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New in spot 2.9 (2020-04-30)
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  Command-line tools:

  - When the --check=stutter-sensitive-example option is passed to
    tools like ltl2tgba, autfilt, genaut, or ltldo, the produced
    automata are checked for stutter-invariance (as in the
    --check=stutter-invariant case), additionally a proof of
    stutter-sensitiveness is provided as two stutter-equivalent words:
    one accepted, and one rejected.  These sample words are printed in
    the HOA output.

    % ltl2tgba --check=stutter-sensitive-example Xa | grep word:
    spot-accepted-word: "!a; cycle{a}"
    spot-rejected-word: "!a; !a; cycle{a}"

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  - autfilt learned the --partial-degeneralize option, to remove
    conjunctions of Inf, or disjunction of Fin that appears in
    arbitrary conditions.

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  - ltlfilt and autfilt learned a --nth=RANGE (a.k.a. -N) option to
    select a range of their input formulas or automata (assuming a
    1-based numbering).

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  - When running translators, ltlcross will now display {names} when
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    supplied.

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  - ltlcross is now using the generic emptiness check procedure
    introduced in Spot 2.7, as opposed to removing Fin acceptance
    before using a classical emptiness check.

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  - ltlcross learned a --save-inclusion-products=FILENAME option.  Its
    use cases are probably quite limited.  We use that to generate
    benchmarks for our generic emptiness check procedure.

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  - ltlsynt --algo=lar uses the new version of to_parity() mentionned
    below.  The old version is available via --algo=lar.old

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  - ltlsynt learned --csv=FILENAME, to record some statistics about
    the duration of its different phases.

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  - The dot printer is now automatically using rectangles with rounded
    corners for automata states if one state label have five or more
    characters.  This saves space with very long labels.  Use --dot=c,
    --dot=e, or --dot=E to force the use of Circles, Ellipses, or
    rEctangles.

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  Library:

  - Historically, Spot only supports LTL with infinite semantics
    so it had automatic simplifications reducing X(1) and X(0) to
    1 and 0 whenever such formulas are constructed.  This
    caused issues for users of LTLf formulas, where it is important
    to distinguish a "weak next" (for which X(1)=1 but X(0)!=0) from
    a "strong next" (for which X(0)=0 but X(1)!=1).

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    To accommodate this, this version introduces a new operator
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    op::strong_X in addition to the existing op::X (whose
    interpretation is now weak in LTLf).  The syntax for the strong
    next is X[!] as a reference to the PSL syntax (where the strong
    next is written X!).

    Trivial simplification rules for X are changed to just
       X(1) = 1       (and not X(0)=0 anymore)
    while we have
       X[!]0 = 0

    The X(0)=0 and X[!]1=1 reductions are now preformed during LTL
    simplification, not automatically.  Aside from the from_ltlf()
    function, the other functions of the library handle X and X[!] in
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    the same way, since there is no difference between X and X[!] over
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    infinite words.

    Operators F[n:m!] and G[n:m!] are also supported as strong
    variants of F[n:m] and G[n:m], but those four are only implemented
    as syntactic sugar.

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  - acc_cond::acc_code::parity(bool, bool, int) was not very readable,
    as it was unclear to the reader what the boolean argument meant.
    The following sister functions can now improve readability:
       parity_max(is_odd, n) = parity(true, is_odd, n)
       parity_max_odd(n) = parity_max(true, n)
       parity_max_even(n) = parity_max(false, n)
       parity_min(is_odd, n) = parity(false, is_odd, n)
       parity_min_odd(n) = parity_min(true, n)
       parity_min_even(n) = parity_min(false, n)

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  - partial_degeneralize() is a new function performing partial
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    degeneralization to get rid of conjunctions of Inf terms, or
    disjunctions of Fin terms in acceptance conditions.
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  - simplify_acceptance_here() and simplify_acceptance() learned to
    simplify subformulas like Fin(i)&Fin(j) or Inf(i)|Inf(j), or some
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    more complex variants of those.  If i is uniquely used in the
    acceptance condition, these become respectively Fin(i) or Inf(i),
    and the automaton is adjusted so that i also appears where j
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    appeared.

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  - acc_code::unit_propagation() is a new method for performing unit
    propagation in acceptance condition.  E.g.  Fin(0) | (Inf(0) &
    Inf(1)) becomes Fin(0) | Inf(1).  This is now called by
    simplify_acceptance_here().

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  - propagate_marks_vector() and propagate_marks_here() are helper
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    functions for propagating marks on the automaton: ignoring
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    self-loops and out-of-SCC transitions, marks common to all the
    input transitions of a state can be pushed to all its outgoing
    transitions, and vice-versa.  This is repeated until a fix point
    is reached.  propagate_marks_vector() does not modify the
    automaton and returns a vector of the acc_cond::mark_t that should
    be on each transition; propagate_marks_here() actually modifies
    the automaton.
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  - rabin_to_buchi_if_realizable() is a new variant of
    rabin_to_buchi_maybe() that converts a Rabin-like automaton into a
    Büchi automaton only if the resulting Büchi automaton can keep the
    same transition structure (where the ..._maybe() variant would
    modify the Rabin automaton if needed).

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  - to_parity() has been rewritten. It now combines several strategies
    for paritizing automata with any acceptance condition.
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  - relabel_bse(), used by ltlfilt --relabel-bool, is now better at
    dealing with n-ary operators and isolating subsets of operands
    that can be relabeled as a single term.

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  - print_dot()'s default was changed to use circles for automata with
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    fewer than 10 unamed states, ellipses for automata with up to 1000
    unamed states (or named states with up to 4 characters), and
    rounded rectangles otherwise.  Rectangles are also used for
    automata with acceptance bullets on states.  The new "E" option
    can be used to force rectangles in all situations.

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  - The generic emptiness check has been slightly improved (doing
    fewer recursive calls in the worst case).

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  Backward-incompatible changes:

  - iar() and iar_maybe() have been moved from
    spot/twaalgos/rabin2parity.hh spot/twaalgos/toparity.hh and marked
    as deprecated, they should be replaced by to_parity().  In case
    the input is Rabin-like or Streett-like, to_parity() should be at
    least as good as iar().
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  - The twa_graph::is_alternating() and digraph::is_alternating() methods,
    deprecated in Spot 2.3.1 (2017-02-20), have been removed.

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  Bugs fixed:

  - Relabeling automata could introduce false edges.  Those are now
    removed.

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  - Emptiness checks, and scc_info should now ignore edges labeled
    with false.

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  - relabel_bse() could incorrectly relabel Boolean subformulas that
    had some atomic propositions in common.

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New in spot 2.8.7 (2020-03-13)
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  Bugs fixed:

  - Building a product between two complete automata where one operand
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    had false acceptance could create an incomplete automaton
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    incorrectly tagged as complete, causing the print_hoa() function
    to raise an exception.

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  - autfilt --uniq was not considering differences in acceptance
    conditions or number of states when discarding automata.

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  - In an automaton with acceptance condition containing some Fin, and
    whose accepting cycle could be reduced to a self-loop in an
    otherwise larger SCC, the generation of an accepting run could be
    wrong.  This could in turn cause segfaults or infinite loops while
    running autcross or autfilt --stats=%w.

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  - The generic emptiness check used a suboptimal selection of "Fin"
    to remove, not matching the correct line in our ATVA'19 paper.
    This could cause superfluous recursive calls, however benchmarks
    have shown the difference to be insignificant in practice.

  - The sl(), and sl2() functions for computing the "self-loopization"
    of an automaton, and used for instance in algorithms for computing
    proof of stutter-sensitiveness (e.g., in our web application),
    were incorrect when applied on automata with "t" acceptance (or
    more generaly, any automaton where a cycle without mark is
    accepting).

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  - ltlcross was not diagnosing write errors associated to
    options --grind=FILENAME and --save-bogus=FILENAME.

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New in spot 2.8.6 (2020-02-19)
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  Bugs fixed:

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  - calling spot.translate() with two conflicting preferences like
    spot.translate(..., 'det', 'any') triggered a syntax error in the
    Python code to handle this error.

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  - degeneralize_tba() was incorrectly not honnoring the "skip_level"
    optimization after creating an accepting transition; as a
    consequence, some correct output could be larger than necessary
    (but still correct).

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  - Some formulas with Boolean sub-formulas equivalent to true or
    false could be translated into automata with false labels.

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  - The HOA printer would incorrectly output the condition such
    'Fin(1) & Inf(2) & Fin(0)' as 'Fin(0) | Fin(1) & Inf(2)'
    due to a bug in the is_generalized_rabin() matching function.

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New in spot 2.8.5 (2020-01-04)
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  Bugs fixed:

  - ltl2tgba -B could return automata with "t" acceptance, instead
    of the expected Inf(0).

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  - twa_graph::merge_edges() (a.k.a. autfilt --merge-transitions) was
    unaware that merging edges can sometimes transform a
    non-deterministic automaton into a deterministic one, causing the
    following unexpected diagnostic:
    "print_hoa(): automaton is universal despite prop_universal()==false"
    The kind of non-deterministic automata where this occurs is not
    naturally produced by any Spot algorithm, but can be found for
    instance in automata produced by Goal 2015-10-18.

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New in spot 2.8.4 (2019-12-08)
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  Bugs fixed:

  - The Rabin-to-Büchi conversion could misbehave when applied to
    Rabin-like acceptance with where some pairs have missing Inf(.)
    (e.g. Fin(0)|(Inf(1)&Fin(2))) and when some of the SCCs do not
    visit the remaining Inf(.).  This indirectly caused the
    complementation algorithm to produce incorrect results on such
    inputs, causing false positives in ltlcross and autcross.

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  - Work around a small difference between Python 3.7 and 3.8, causing
    spurious failures of the test suite.

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New in spot 2.8.3 (2019-11-06)
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  Build:

  - Minor portabilities improvements with C++17 deprecations.
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  Python:

  - Doing "import spot.foo" will now load any spot-extra/foo.py on
    Python's search path.  This can be used to install third-party
    packages that want to behave as plugins for Spot.

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  - Work around GraphViz bug #1605 in Jupyter notebooks.
    https://gitlab.com/graphviz/graphviz/issues/1605

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New in spot 2.8.2 (2019-09-27)
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  Command-line tools:

  - ltl2tgba and ltldo learned a --negate option.

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  Bugs fixed:

  - Calling "autfilt --dualize" on an alternating automaton with
    transition-based acceptance and universal initial states would
    fail with "set_init_state() called with nonexisting state".
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  - The numbering of nodes in the AIGER output of ltlsynt was
    architecture dependent.

  - Various compilation issues.  In particular, this release is the
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    first one that can be compiled (and pass tests) on a Raspberry PI.
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New in spot 2.8.1 (2019-07-18)
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  Command-line tools:

  - genltl learned --pps-arbiter-standard and --pps-arbiter-strict.

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  - ltlcross and autcross have learned a new option -q (--quiet) to
    remain quiet until an error is found.  This helps for instance in
    scenarios where multiple instances of ltlcross/autcross are run in
    parallel (using "xargs -P" or "GNU Parallel", for instance).  See
    https://spot.lrde.epita.fr/ltlcross.html#parallel for examples.

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  Bugs fixed:

  - When complement() was called with an output_aborter, it could
    return an alternating automaton on large automata.  This in turn
    caused ltlcross to emit errors like "remove_alternation() only
    works with weak alternating automata" or "product() does not
    support alternating automata".
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  - Work around Emacs bug #34341 when rebuilding documentation on a
    system with Emacs <26.3, GNU TLS >= 3.6, and without ESS or with
    an obsolete org-mode installed.

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New in spot 2.8 (2019-07-10)
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  Command-line tools:

  - autfilt learned --highlight-accepting-run=NUM to highlight some
    accepting run with color NUM.

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  - ltldo, ltlcross, and autcross are now preferring posix_spawn()
    over fork()+exec() when available.

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  - ltlcross has new options --determinize-max-states=N and
    --determinize-max-edges=M to restrict the use of
    determinization-based complementation to cases where it produces
    automata with at most N states and M edges.  By default
    determinization is now attempted up to 500 states and 5000
    edges.  This is an improvement over the previous default where
    determinization-based complementation was not performed at all,
    unless -D was specified.

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  - ltlcross will now skip unnecessary cross-checks and
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    consistency-checks (they are unnecessary when all automata
    could be complemented and statistics were not required).

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  - genaut learned --m-nba=N to generate Max Michel's NBA family.
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    (NBAs with N+1 states whose determinized have at least N! states.)

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  - Due to some new simplification of parity acceptance, the output of
    "ltl2tgba -P -D" is now using a minimal number of colors.  This
    means that recurrence properties have an acceptance condition
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    among "Inf(0)", "t", or "f", and persistence properties have an
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    acceptance condition among "Fin(0)", "t", or "f".

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  - ltldo and ltlcross learned shorthands to call the tools ltl2na,
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    ltl2nba, and ltl2ngba from Owl 19.06.  Similarly, autcross learned
    a shorthand for Owl's dra2dpa.
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  Documentation:

  - https://spot.lrde.epita.fr/tut90.html is a new file that explains
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    the purpose of the spot::bdd_dict object.
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  Library:

  - Add generic_accepting_run() as a variant of generic_emptiness_check() that
    returns an accepting run in an automaton with any acceptance condition.

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  - twa::accepting_run() and twa::intersecting_run() now work on
    automata using Fin in their acceptance condition.
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  - simulation-based reductions have learned a trick that sometimes
    improve transition-based output when the input is state-based.
    (The automaton output by 'ltl2tgba -B GFa | autfilt --small' now
    has 1 state instead of 2 in previous versions.  Similarly,
    'ltldo ltl2dstar -f 'GFa -> GFb' | autfilt --small' produces 1
    state instead of 4.)

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  - simulation-based reductions has learned another trick to better
    merge states from transient SCCs.
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  - acc_cond::top_disjuncts() and acc_cond::top_conjuncts() can be
    used to split an acceptance condition on the top-level & or |.
    These methods also exist in acc_cond::acc_code.
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  - minimize_obligation() learned to work on very weak automata even
    if the formula or negated automaton are not supplied.  (This
    allows "autfilt [-D] --small" to minimize very-weak automata
    whenever they are found to represent obligation properties.)

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  - There is a new spot::scc_and_mark_filter object that simplifies the
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    creation of filters to restrict spot::scc_info to some particular
    SCC while cutting new SCCs on given acceptance sets.  This is used
    by spot::generic_emptiness_check() when processing SCCs
    recursively, and makes it easier to write similar code in Python.

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  - print_dot has a new option 'g', to hide edge labels.  This is
    helpful to display automata as "graphs", e.g., when illustrating
    algorithms that do not care about labels.

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  - A new complement() function returns complemented automata with
    unspecified acceptance condition, allowing different algorithms to
    be used.  The output can be alternating only if the input was
    alternating.
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  - There is a new class output_aborter that is used to specify
    upper bounds on the size of automata produced by some algorithms.
    Several functions have been changed to accept an output_aborter.
    This includes:
      * tgba_determinize()
      * tgba_powerset()
      * minimize_obligation()
      * minimize_wdba()
      * remove_alternation()
      * product()
      * the new complement()
      * the postprocessor class, via the "det-max-state" and
        "det-max-edges" options.

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  - SVA's first_match operator can now be used in SERE formulas and
    that is supported by the ltl_to_tgba_fm() translation.  See
    doc/tl/tl.pdf for the semantics.  *WARNING* Because this adds a
    new operator, any code that switches over the spot::op type may
    need a new case for op::first_match.  Furthermore, the output of
    "randltl --psl" will be different from previous releases.

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  - The parser for SERE learned to recognize the ##n and ##[i:j]
    operators from SVA.  So {##2 a ##0 b[+] ##1 c ##2 e} is another
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    way to write {[*2];a:b[+];c;1;e}.  The syntax {a ##[i:j] b} is
    replaced in different ways depending on the values of i, a, and b.
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    The formula::sugar_delay() function implements this SVA operator in
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    terms of the existing PSL operators.  ##[+] and ##[*] are sugar
    for ##[1:$] and ##[0:$].
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  - The F[n:m] and G[n:m] operators introduced in Spot 2.7 now
    support the case where m=$.

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  - spot::relabel_apply() makes it easier to reverse the effect
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    of spot::relabel() or spot::relabel_bse() on formula.

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  - The LTL simplifier learned the following rules:
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    F(G(a | Fb)) = FGa | GFb  (if option "favor_event_univ")
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    G(F(a | Gb)) = GFa | FGb  (if option "favor_event_univ")
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    F(G(a & Fb) = FGa & GFb   (unless option "reduce_size_strictly")
    G(F(a & Gb)) = GFa & FGb  (unless option "reduce_size_strictly")
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    GF(f) = GF(dnf(f))        (unless option "reduce_size_strictly")
    FG(f) = FG(cnf(f))        (unless option "reduce_size_strictly")
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    (f & g) R h = f R h       if h implies g
    (f & g) M h = f M h       if h implies g
    (f | g) W h = f W h       if g implies h
    (f | g) U h = f U h       if g implies h
    Gf | F(g & eventual) = f W (g & eventual)    if !f implies g
    Ff & G(g | universal) = f M (g | universal)  if f implies !g
    f U (g & eventual) = F(g & eventual)         if !f implies g
    f R (g | universal) = G(g | universal)       if f implies !g
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  - cleanup_parity() and colorize_parity() were cleaned up a bit,
    resulting in fewer colors used in some cases.  In particular,
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    colorize_parity() learned that coloring transient edges does not
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    require the introduction of a new color.

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  - A new reduce_parity() function implements and generalizes the
    algorithm for minimizing parity acceptance by Carton and Maceiras
    (Computing the Rabin index of a parity automaton, 1999).  This is
    a better replacement for cleanup_parity() and colorize_parity().
    See https://spot.lrde.epita.fr/ipynb/parity.html for examples.

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  - The postprocessor and translator classes are now using
    reduce_parity() for further simplifications.

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  - The code for checking recurrence/persistence properties can also
    use the fact the reduce_parity() with return "Inf(0)" (or "t" or
    "f") for deterministic automata corresponding to recurrence
    properties, and "Fin(0)" (or "t" or "f") for persistence
    properties.  This can be altered with the SPOT_PR_CHECK
    environment variable.

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  Deprecation notices:
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  - The virtual function twa::intersecting_run() no longer takes a
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    second "from_other" Boolean argument.  This is a backward
    incompatibility only for code that overrides this function in a
    subclass.  For backward compatibility with programs that simply
    call this function with two argument, a non-virtual version of the
    function has been introduced and marked as deprecated.

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  - The spot::acc_cond::format() methods have been deprecated.  These
    were used to display acceptance marks, but acceptance marks are
    unrelated to acceptance conditions, so it's better to simply print
    marks with operator<< without this extra step.

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  Bugs fixed:

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  - The gf_guarantee_to_ba() is relying on an in-place algorithm that
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    could produce a different number of edges for the same input in
    two different transition order.

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  - A symmetry-based optimization of the LAR algorithm performed in
    spot::to_parity() turned out to be incorrect.  The optimization
    has been removed.  "ltlsynt --algo=lar" is the only code using
    this function currently.

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New in spot 2.7.5 (2019-06-05)
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  Build:

  - Although the Python bindings in this release are still done with
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    Swig3.0, the code has been updated to be compatible with Swig4.0.
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  Library:

  - print_dot will replace labels that have more 2048 characters by a
    "(label too long)" string.  This works around a limitation of
    GraphViz that aborts when some label exceeds 16k characters, and
    also helps making large automata more readable.
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  Bugs fixed:

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  - spot::translator was not applying Boolean sub-formula rewriting
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    by default unless a spot::option_map was passed.  This caused some
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    C++ code for translating certain formulas to be noticeably slower
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    than the equivalent call to the ltl2tgba binary.

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  - The remove_ap algorithm was preserving the "terminal property" of
    automata, but it is possible that a non-terminal input produces a
    terminal output after some propositions are removed.

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New in spot 2.7.4 (2019-04-27)
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  Bugs fixed:

  - separate_sets_here() (and therefore autfilt --separate-sets) could
    loop infinitely on some inputs.

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  - In some situation, ltl2tgba -G could abort with
    "direct_simulation() requires separate Inf and Fin sets".  This
    was fixed by teaching simulation-based reductions how to deal
    with such cases.

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  - The code for detecting syntactically stutter-invariant PSL
    formulas was incorrectly handling the ";" operator, causing some
    stutter-sensitive formulas to be flagged a stutter-invariant.

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New in spot 2.7.3 (2019-04-19)
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  Bugs fixed:

  - When processing CSV files with MSDOS-style \r\n line endings,
    --stats would output the \r as part of the %> sequence instead
    of ignoring it.

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  - Fix serious typo in remove_alternation() causing incorrect
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    output for some VWAA.  Bug introduced in Spot 2.6.

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  Documentation:

  - Multiple typos and minor updates.

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New in spot 2.7.2 (2019-03-17)

  Python:

  - Improved support for explicit Kripke structures.  It is now
    possible to iterate over a kripke_graph object in a way similar to
    twa_graph.
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  Documentation:

  - A new page shows how to create explicit Kripke structures in C++
    and Python.  See https://spot.lrde.epita.fr/tut52.html
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  - Another new page shows how to deal with LTLf formulas (i.e., LTL
    with finite semantics) and how to translate those.
    See https://spot.lrde.epita.fr/tut12.html
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  Build:

  - Work around a spurious null dereference warning when compiling
    with --coverage and g++ 8.3.0-3 from Debian unstable.

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New in spot 2.7.1 (2019-02-14)
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  Build:
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  - Work around GCC bug #89303 that causes memory leaks and std::weak_bad_ptr
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    exceptions when Spot is compiled with the version of g++ 8.2 currently
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    distributed by Debian unstable (starting with g++ 8.2.0-15).
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  Python:

  - The following methods of spot::bdd_dict are now usable in Python when
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    fine control over the lifetime of associations between BDD variables
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    and atomic propositions is needed.
    - register_proposition(formula, for_me)
    - register_anonymous_variables(count, for_me)
    - register_all_propositions_of(other, for_me)
    - unregister_all_my_variables(for_me)
    - unregister_variable(var, for_me)

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  - Better support for explicit Kripke structures:
    - the kripke_graph type now has Python bindings
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    - spot.automaton() and spot.automata() now support a want_kripke=True
      to return a kripke_graph
      See the bottom of https://spot.lrde.epita.fr/ipynb/ltsmin-dve.html
      for some examples.

  Library:

  - Printing Kripke structures via print_hoa() will save state names.

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  - kripke_graph_ptr objects now honor any "state-names" property
    when formatting states.
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  Bugs fixed:

  - The print_dot_psl() function would incorrectly number all but the
    first children of commutative n-ary operators: in this case no
    numbering was expected.
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  - std::out_of_range C++ exceptions raised from Python code are now
    converted into IndexError Python exceptions (instead of aborting
    the program).

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  - The LTL parser would choke on carriage returns when command-line
    tools such as ltlfilt, ltlcross, or ltl2tgba were run on files of
    formulas with MS-DOS line endings.

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  - The core translation for unambiguous automata was incorrectly
    tagging some non-weak automata as weak.

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  - The product_susp() function used to multiply an automaton with a
    suspendable automaton could incorrectly build transition-based
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    automata when multiplying two state-based automata.  This caused
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    ltl2tgba to emit error messages such as: "automaton has
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    transition-based acceptance despite prop_state_acc()==true".
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New in spot 2.7 (2018-12-11)
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  Command-line tools:

  - ltlsynt now has three algorithms for synthesis:
    --algo=sd     is the historical one. The automaton of the formula
                  is split to separate inputs and outputs, then
                  determinized (with Safra construction).
    --algo=ds     the automaton of the formula is determinized (Safra),
                  then split to separate inputs and outputs.
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    --algo=lar    translate the formula to a deterministic automaton
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                  with an arbitrary acceptance condition, then turn it
                  into a parity automaton using LAR, and split it.
    In all three cases, the obtained parity game is solved using
    Zielonka algorithm. Calude's quasi-polynomial time algorithm has
    been dropped as it was not used.

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  - ltlfilt learned --liveness to match formulas representing liveness
    properties.
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  - the --stats= option of tools producing automata learned how to
    tell if an automaton uses universal branching (%u), or more
    precisely how many states (%[s]u) or edges (%[e]u) use universal
    branching.

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  Python:

  - spot.translate() and spot.postprocess() now take an xargs=
    argument similar to the -x option of ltl2tgba and autfilt, making
    it easier to fine tune these operations.  For instance
        ltl2tgba 'GF(a <-> XXa)' --det -x gf-guarantee=0
    would be written in Python as
        spot.translate('GF(a <-> XXa)', 'det', xargs='gf-guarantee=0')
    (Note: those extra options are documented in the spot-x(7) man page.)

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  - spot.is_generalized_rabin() and spot.is_generalized_streett() now return
    a tuple (b, v) where b is a Boolean, and v is the vector of the sizes
    of each generalized pair.  This is a backward incompatible change.

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  Library:

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  - The LTL parser learned syntactic sugar for nested ranges of X
    using the X[n], F[n:m], and G[n:m] syntax of TSLF.  (These
    correspond to the next!, next_e!, and next_a! operators of PSL,
    but we do not support those under these names currently.)

      X[6]a = XXXXXXa
      F[2:4]a = XX(a | X(a | Xa))
      G[2:4]a = XX(a & X(a & Xa))

    The corresponding constructors (for C++ and Python) are
      formula::X(unsigned, formula)
      formula::F(unsigned, unsigned, formula)
      formula::G(unsigned, unsigned, formula)
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  - spot::unabbreviate(), used to rewrite away operators such as M or
    W, learned to use some shorter rewritings when an argument (e) is
    a pure eventuality or (u) is purely universal:

      Fe = e
      Gu = u
      f R u = u
      f M e = F(f & e)
      f W u = G(f | u)

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  - The twa_graph class has a new dump_storage_as_dot() method
    to show its data structure.  This is more conveniently used
    as aut.show_storage() in a Jupyter notebook.  See
    https://spot.lrde.epita.fr/ipynb/twagraph-internals.html

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  - spot::generic_emptiness_check() is a new function that performs
    emptiness checks of twa_graph_ptr (i.e., automata not built
    on-the-fly) with an *arbitrary* acceptance condition.  Its sister
    spot::generic_emptiness_check_scc() can be used to decide the
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    emptiness of an SCC.  This is now used by
    twa_graph_ptr::is_empty(), twa_graph_ptr::intersects(), and
    scc_info::determine_unknown_acceptance().
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  - The new function spot::to_parity() translates an automaton with
    arbitrary acceptance condition into a parity automaton, based on a
    last-appearance record (LAR) construction.  (It is used by ltlsynt
    but not yet by autfilt or ltl2tgba.)
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  - The new function is_liveness() and is_liveness_automaton() can be
    used to check whether a formula or an automaton represents a
    liveness property.

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  - Two new functions count_univbranch_states() and
    count_univbranch_edges() can help measuring the amount of
    universal branching in alternating automata.

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  Bugs fixed:

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  - translate() would incorrectly mark as stutter-invariant
    some automata produced from formulas of the form X(f...)
    where f... is syntactically stutter-invariant.

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  - acc_cond::is_generalized_rabin() and
    acc_cond::is_generalized_streett() did not recognize the cases
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    where a single generalized pair is used.
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  - The pair of acc_cond::mark_t returned by
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    acc_code::used_inf_fin_sets(), and the pair (bool,
    vector_rs_pairs) by acc_cond::is_rabin_like() and
    acc_cond::is_streett_like() were not usable in Python.
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  - Many object types had __repr__() methods that would return the
    same string as __str__(), contrary to Python usage where repr(x)
    should try to show how to rebuild x.  The following types have
    been changed to follow this convention:
       spot.acc_code
       spot.acc_cond
       spot.atomic_prop_set
       spot.formula
       spot.mark_t
       spot.twa_run (__repr__ shows type and address)
       spot.twa_word (likewise, but _repr_latex_ used in notebooks)

    Note that this you were relying on the fact that Jupyter calls
    repr() to display returned values, you may want to call print()
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    explicitly if you prefer the old representation.
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  - Fix compilation under Cygwin and Alpine Linux, both choking
    on undefined secure_getenv().

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New in spot 2.6.3 (2018-10-17)
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  Bugs fixed:

  - Running "ltl2tgba -B" on formulas of the type FG(safety) would
    unexpectedly use a co-Büchi automaton as an intermediate step.
    This in turn caused "ltl2tgba -U -B" to not produce unambiguous
    automata.

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  - ltl2tgba --low now disables the "gf-guarantee" feature, as
    documented.

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  - ltlfilt's --accept-word and --reject-word options were ignored
    unless used together.

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New in spot 2.6.2 (2018-09-28)
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  Build:

  - We no longer distribute the Python-based CGI script + javascript
    code for the online translator.  Its replacement has its own
    repository: https://gitlab.lrde.epita.fr/spot/spot-web-app/
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  Library:

  - When states are highlighted with more than 8 colors, print_dot()
    will add some extra markers to help distinguishing the colors.
    This helps with the fact that colors 8-15 are lighter versions of
    colors 0-7, and that higher color numbers cycle into this 16-color
    palette.

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  Bugs fixed:

  - exclusive_ap::constrain() (called by autfilt --exclusive-ap=...)
    would incorrectly copy the "universal" property of the input
    automaton, causing print_hoa() to fail.

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  - configure --disable-doxygen would actually enable it.

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  - Fix several warnings emitted by the development version of GCC.
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New in spot 2.6.1  (2018-08-04)
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  Command-line tools:

  - "ltlfilt --suspendable" is now a synonym for
    "ltlfilt --universal --eventual".
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  Bugs fixed:

  - scc_info::split_on_sets() did not correctly register the
    atomic propositions of the returned automata.

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  - The spot::tl_simplifier class could raise an exception while
    attempting to reduce formulas containing unsimplified <->, -> or
    xor, if options nenoform_stop_on_boolean and synt_impl are both
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    set.  (This combination of options is not available from
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    command-line tools.)

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  - The spot::contains(a, b) function introduced in 2.6 was testing
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    a⊆b instead of a⊇b as one would expect.  Unfortunately the
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    documentation was also matching the code, so this is a backward
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    incompatible change, but a short-lived one.
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  - The Python binding of the getter of spot::parsed_formula::f was
    returning a reference instead of a copy, causing issues if the
    reference outlasted the parsed_formula struct.

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New in spot 2.6  (2018-07-04)
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  Command-line tools:

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  - autfilt learned --is-colored to filter automata that use
    exactly one acceptance set per mark or transition.

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  - autfilt learned --has-univ-branching and --has-exist-branching
    to keep automata that have universal branching, or that make
    non-deterministic choices.

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  - autcross' tool specifications now have %M replaced by the name of
    the input automaton.

  - autcross now aborts if there is any parser diagnostic for the
    input automata (previous versions would use the input automaton
    whenever the parser would manage to read something).

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  - ltlcross, ltldo, and autcross learned shorthands to call
    delag, ltl2dra, ltl2dgra, and nba2dpa.

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  - When autfilt is asked to build a Büchi automaton from
    of a chain of many products, as in
    "autfilt -B --product 1.hoa ... --product n.hoa in.hoa"
    then it will automatically degeneralize the intermediate
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    products to avoid exceeding the number of supported
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    acceptance sets.

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  - genltl learned to generate six new families of LTL formulas:
      --gf-equiv-xn=RANGE    GF(a <-> X^n(a))
      --gf-implies-xn=RANGE  GF(a -> X^n(a))
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      --sejk-f=RANGE[,RANGE] f(0,j)=(GFa0 U X^j(b)), f(i,j)=(GFai U
                             G(f(i-1,j)))
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      --sejk-j=RANGE         (GFa1&...&GFan) -> (GFb1&...&GFbn)
      --sejk-k=RANGE         (GFa1|FGb1)&...&(GFan|FGbn)
      --sejk-patterns[=RANGE]   φ₁,φ₂,φ₃ from Sikert et al's [CAV'16]
                             paper (range should be included in 1..3)

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  - genltl --ms-example can now take a two-range argument,
    (as --sejk-f above).

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  Build:

  - ./configure --enable-max-accsets=N let you specify a maximum
    number of acceptance sets that Spot should support.  The default
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    is still 32, but this limit is no longer hard-coded.  Larger values
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    will cost additional memory and time.

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  - We know distribute RPM packages for Fedora 28.  See
    file:///home/adl/git/spot/doc/userdoc/install.html#Fedora

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  Library:

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  - The PSL/LTL simplification routine learned the following:

      q R Xf = X(q R f)  if q is suspendable
      q U Xf = X(q U f)  if q is suspendable
      {SERE;1} = {1}     if {SERE} accepts [*0]
      {SERE;1} = {SERE}  if {SERE} does not accept [*0]

  - gf_guarantee_to_ba() is a specialized construction for translating
    formulas of the form GF(guarantee) to BA or DBA, and
    fg_safety_to_dca() is a specialized construction for translating
    formulas of the form FG(safety) to DCA.  These are generalizations
    of some constructions proposed by J. Esparza, J. Křentínský, and
    S. Sickert (LICS'18).

    These are now used by the main translation routine, and can be
    disabled by passing -x '!gf-guarantee' to ltl2tgba.  For example,
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    here are the size of deterministic transition-based generalized
    Büchi automata constructed from four GF(guarantee) formulas with
    two versions of Spot, and converted to other types of
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    deterministic automata by other tools distributed with Owl 18.06.
    "x(y)" means x states and y acceptance sets.
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                               ltl2tgba -D  delag ltl2dra
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                                2.5   2.6   18.06  18.06
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    -------------------------  -----------  -------------
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    GF(a <-> XXa)               9(1)  4(1)   4(2)   9(4)
    GF(a <-> XXXa)             27(1)  8(1)   8(2)  25(4)
    GF(((a & Xb) | XXc) & Xd)   6(1)  4(1)  16(1)   5(2)
    GF((b | Fa) & (b R Xb))     6(2)  2(1)   3(4)   3(4)

    Note that in the above the automata produced by 'ltl2tgba -D' in
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    version 2.5 were not deterministic (because -D is only a
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    preference).  They are deterministic in 2.6 and other tools.
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  - spot::product() and spot::product_or() learned to produce an
    automaton with a simpler acceptance condition if one of the
    argument is a weak automaton.  In this case the resulting
    acceptance condition is (usually) that of the other argument.

  - spot::product_susp() and spot::product_or_susp() are new
    functions for building products between an automaton A and
    a "suspendable" automaton B.  They are also optimized for
    the case that A is weak.

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  - When 'generic' acceptance is enabled, the translation routine will
    split the input formula on Boolean operators into components that
    are syntactically 'obligation', 'suspendable', or 'something
    else'.  Those will be translated separately and combined with
    product()/product_susp().  This is inspired by the ways things are
    done in ltl2dstar or delag, and can be disabled by passing option
    -xltl-split=0, as in ltl2tgba -G -D -xltl-split=0.  Here are the
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    sizes of deterministic automata (except for ltl3tela which produces
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    non-deterministic automata) produced with generic acceptance
    using two versions of ltl2tgba and other tools for reference.

                                             ltl2tgba -DG  delag  ltl3tela
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                                              2.5    2.6   18.06    1.1.2
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                                             ------------ ------- --------
    FGa0&GFb0                                 2(2)   1(2)   1(2)    1(2)
    (FGa1&GFb1)|FGa0|GFb0                    16(6)   1(4)   1(4)    1(9)
    (FGa2&GFb2)|((FGa1|GFb1)&FGa0&GFb0)      29(8)   1(6)   1(6)    3(11)
    FGa0|GFb0                                 5(4)   1(2)   1(2)    1(5)
    (FGa1|GFb1)&FGa0&GFb0                     8(4)   1(4)   1(4)    1(7)
    (FGa2|GFb2)&((FGa1&GFb1)|FGa0|GFb0)     497(14)  1(6)   1(6)    1(14)
    GFa1 <-> GFz                              4(6)   1(3)   1(4)    1(7)
    (GFa1 & GFa2) <-> GFz                     8(4)   1(3)   1(6)    3(10)
    (GFa1 & GFa2 & GFa3) <-> GFz             21(4)   1(4)   1(8)    3(13)
    GFa1 -> GFz                               5(4)   1(2)   1(2)    1(5)
    (GFa1 & GFa2) -> GFz                     12(4)   1(3)   1(3)    1(6)
    (GFa1 & GFa2 & GFa3) -> GFz              41(4)   1(4)   1(4)    1(7)
    FG(a|b)|FG(!a|Xb)                         4(2)   2(2)   2(2)    2(3)
    FG(a|b)|FG(!a|Xb)|FG(a|XXb)              21(2)   4(3)   4(3)    4(4)
    FG(a|b)|FG(!a|Xb)|FG(a|XXb)|FG(!a|XXXb) 170(2)   8(4)   8(4)    8(5)

  - For 'parity' output, the 'ltl-split' optimization just separates
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    obligation sub-formulas from the rest, where a determinization is
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    still performed.
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                                 ltl2tgba -DP  ltl3dra  ltl2dpa
                                   2.5   2.6    0.2.3    18.06
                                -------------- -------  -------
    FGp0 & (Gp1 | XFp2)           6(2)  4(1)     4(1)     4(2)
    G!p0 | F(p0 & (!p1 W p2))     5(2)  4(2)     n/a      5(2)
    (p0 W XXGp0) & GFp1 & FGp2    6(2)  5(2)     n/a      6(3)
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    (The above just show a few cases that were improved.  There are
    many cases where ltl2dpa still produces smaller automata.)

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  - The automaton postprocessor will now simplify acceptance
    conditions more aggressively, calling spot::simplify_acceptance()
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    or spot::cleanup_acceptance() depending on the optimization level.
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  - print_dot(), used to print automata in GraphViz's format,
    underwent several changes:
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    * option "a", for printing the acceptance condition, is now
      enabled by default.  Option "A", introduced in Spot 2.4, can be
      used to hide the acceptance condition in case you do not want
      it.  This change of course affects the --dot option of all the
      command-line tools, as well as the various way to display
      automata using the Python bindings.

    * when option "1" is used to hide state names and force the
      display of state numbers, the actual state names is now moved to
      the "tooltip" field of the state.  The SVG files produced by
      "dot -Tsvg" will show those as popups.  This is also done for
      state labels of Kripke structures.

    * the output digraph is now named using the name of the automaton
      if available, or the empty string otherwise.  (Previous versions
      used to call all digraphs "G".)  This name appears as a tooltip
      in SVG figures when the mouse is over the acceptance condition.

    * a new option "u" hides "true states" behind "exiting
      transitions".  This can be used to display alternating automata
      in a way many people expect.

    * a new option "K" cancels the effect of "k" (which uses state
      labels whenever possible).  This is most useful when one want to
      force transition-labeling of a Kripke structure, where "k" is
      usually the default.

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  - spot::twa_graph::merge_states() is a new method that merges states
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    with the exact same outgoing edges. As it performs no reordering
    of the edges, it is better to call it when you know that the edges
    in the twa_graph are sorted (e.g. after a call to merge_edges()).
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  - spot::twa_graph::purge_unreachable_states() now takes a function
    which is called with the new numbering of states. This is useful
    to update an external structure that references states of the twa
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    that we want to purge.

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  - spot::scc_filter() now automatically turns automata marked as
    inherently-weak into weak automata with state-based acceptance.
    The acceptance condition is set to Büchi unless the input had
    co-Büchi or t acceptance.  spot::scc_filter_states() will pass
    inherently-weak automata to spot::scc_filter().

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  - spot::cleanup_parity() and spot::cleanup_parity_here() are smarter
    and now remove from the acceptance condition the parity colors
    that are not used in the automaton.
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  - spot::contains() and spot::are_equivalent() can be used to
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    check language containment between two automata or formulas.
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    They are most welcome in Python, since we used to redefine
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    them every now and them.  Some examples are shown in
    https://spot.lrde.epita.fr/ipynb/contains.html

  - aut1->exclusive_word(aut2) is a new method that returns a word
    accepted by aut1 or aut2 but not both.  The exclusive_run()
    variant will return.  This is useful when comparing automata and
    looking for differences.  See also
    https://spot.lrde.epita.fr/ipynb/contains.html
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  - spot::complement_semidet(aut) is a new function that returns the
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    complement of aut, where aut is a semi-deterministic automaton. The
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    function uses the NCSB complementation algorithm proposed by
    F. Blahoudek, M. Heizmann, S. Schewe, J. Strejček, and MH. Tsai
    (TACAS'16).

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  - spot::remove_alternation() was slightly improved on very-weak
    alternating automata: the labeling of the outgoing transitions in
    the resulting TGBA makes it more likely that simulation-based
    reductions will reduce it.

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  - When applied to automata that are not WDBA-realizable,
    spot::minimize_wdba() was changed to produce an automaton
    recognizing a language that includes the original one.  As a
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    consequence spot::minimize_obligation() and
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    spot::is_wdba_realizable() now only need one containment check
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    instead of two.
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  - Slightly improved log output for the SAT-based minimization
    functions.  The CSV log files now include an additional column
    with the size of the reference automaton, and they now have a
    header line.  These log files give more details and are more
    accurate in the case of incremental SAT-solving.
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  Python:

  - New spot.jupyter package.  This currently contains a function for
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    displaying several arguments side by side in a Jupyter notebook.
    See https://spot.lrde.epita.fr/ipynb/alternation.html for some
    examples.
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  - Strings are now implicitly converted into formulas when passed
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    as arguments to functions that expect formulas.  Previously this
    was done only for a few functions.

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  - The Python bindings for sat_minimize() now have display_log and
    return_log options; these are demonstrated on the new
    https://spot.lrde.epita.fr/ipynb/satmin.html page.

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  Bugs fixed:

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  - Python *.py and *.so files are now always installed into the same
    directory.  This was an issue on systems like Fedora that separate
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    platform-specific packages from non-platform-specific ones.
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  - print_dot() will correctly escape strings containing \n in HTML
    mode.

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  - The HOA parser will now accept Alias: declarations that occur
    before AP:.

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  - The option --allow-dups of randltl now works properly.

  - Converting generalized-co-Büchi to Streett using dnf_to_nca()
    could produce bogus automata if the input had rejecting SCCs.
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  Deprecation notices:
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  - The type spot::acc_cond::mark_t has been overhauled and uses
    a custom bit-vector to represent acceptance sets instead of
    storing everything in a "unsigned int".  This change is
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    to accommodate configure's --enable-max-accsets=N option and
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    has several effect:

    * The following shortcuts are deprecated:
        acc_cond::mark_t m1 = 0U;
        acc_cond::mark_t m2 = -1U;
      instead, use:
        acc_cond::mark_t m1 = {};
        acc_cond::mark_t m2 = acc_cond::mark_t::all();

    * acc_cond::mark_t::value_t is deprecated.  It is now only
      defined when --enable-max-accsets=32 (the default) and
      equal to "unsigned" for backward compatibility reasons.

  Backward incompatibilities:

  - Functions spot::parity_product() and spot::parity_product_or()
    were removed.  The code was unused, hard to maintain, and bogus.

  - The output of print_dot() now include the acceptance condition.
    Add option "A" (supported since version 2.4) to cancel that.

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  - Because genltl now supports LTL pattern with two arguments, using
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    --format=%L may output two comma-separated integer.  This is an
    issue if you used to produce CSV files using for instance:
      genltl --format='%F,%L,%f' ...
    Make sure to quote %L to protect the potential commas:
      genltl --format='%F,"%L",%f' ...

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  - In Spot 2.5 and prior running "ltl2tgba --generic --det" on some
    formula would attempt to translate it as deterministic TGBA or
    determinize it into an automaton with parity acceptance.  Version
    2.5 introduced --parity to force parity acceptance on the output.
    This version finally gives --generic some more natural semantics:
    any acceptance condition can be used.

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New in spot 2.5.3 (2018-04-20)
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  Bugs fixed:

  - "autfilt --cobuchi --small/--det" would turn a transition-based
    co-Büchi automaton into a state-based co-Büchi.

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  - Fix cryptic error message from Python's spot.translate() and
    spot.postprocess() when supplying conflicting arguments.

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  - "autfilt -B --sat-minimize" was incorrectly producing
    transition-based automata.

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  - Using spot.automata("cmd...|") to read just a few automata out of
    an infinite stream would not properly terminate the command.

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  - The is_unambiguous() check (rewritten in Spot 2.2) could mark some
    unambiguous automata as ambiguous.

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New in spot 2.5.2 (2018-03-25)
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  Bugs fixed:

  - acc_cond::is_generalized_rabin() and
    acc_cond::is_generalized_streett() would segfault on weird
    acceptance conditions such as "3 t" or "3 f".

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  - remove_fin() and streett_to_generalized_buchi() should never
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    return automata with "f" acceptance.

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  - "autfilt --acceptance-is=Fin-less" no longer accept automata
    with "f" acceptance.

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  - twa_run methods will now diagnose cases where the cycle is
    unexpectedly empty instead of segfaulting.

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  - spot::closure(), used by default for testing stutter-invariance,
    was using an optimization incorrect if the acceptance condition
    had some Fin(x).  Consequently stutter-invariance tests for
    automata, for instance with "autfilt --is-stutter-invariant",
    could be to be wrong (even if the input does not use Fin(x), its
    complement, used in the stutter-invariance test likely will).
    Stutter-invariance checks of LTL formulas are not affected.

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New in spot 2.5.1 (2018-02-20)
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  Library:

  - iar() and iar_maybe() now also handle Streett-like conditions.

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  Bugs fixed:

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  - iar() and iar_maybe() properly handle Rabin-like conditions.

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  - streett_to_generalized_buchi() could produce incorrect result on
    Streett-like input with acceptance like (Inf(0)|Fin(1))&Fin(1)
    where some Fin(x) is used both with and without a paired Fin(y).
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  - is_generalized_rabin() had a typo that caused some non-simplified
    acceptance conditions like Fin(0)|(Fin(0)&Inf(1)) to be
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    incorrectly detected as generalized-Rabin 2 0 1 and then output
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    as Fin(0)|(Fin(1)&Inf(2)) instead.  Likewise for
    is_generalized_streett

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  - the conversion from Rabin to Büchi was broken on Rabin-like
    acceptance condition where one pair used Fin(x) and another pair
    used Inf(x) with the same x.  Unfortunately, this situation could
    also occur as a side effect of simplifying the acceptance
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    condition (by merging identical set) of some automaton prior to
    converting it to Büchi.
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  - dnf_to_dca() was mishandling some Rabin-like input.

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New in spot 2.5 (2018-01-20)
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  Build:

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  - We no longer distribute the doxygen-generated documentation in
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    the tarball of Spot to save space.  This documentation is still
    available online at https://spot.lrde.epita.fr/doxygen/.  If you
    want to build a local copy you can configure Spot with
    --enable-doxygen, or simply run "cd doc && make doc".

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  - All the images that illustrate the documentation have been
    converted to SVG, to save space and improve quality.

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  Tools:

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  - ltlsynt is a new tool for synthesizing a controller from LTL/PSL
    specifications.

  - ltlcross learned --reference=COMMANDFMT to specify a translator
    that should be trusted.  Doing so makes it possible to reduce the
    number of tests to be performed, as all other translators will be
    compared to the reference's output when available.  Multiple
    reference can be given; in that case other tools are compared
    against the smallest reference automaton.

  - autcross, ltlcross, and ltldo learned --fail-on-timeout.

  - ltl2tgba, autfilt, and dstar2tgba have some new '--parity' and
    '--colored-parity' options to force parity acceptance on the
    output.  Different styles can be requested using for instance
    --parity='min odd' or --parity='max even'.

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  - ltl2tgba, autfilt, and dstar2tgba have some new '--cobuchi' option
    to force co-Büchi acceptance on the output.  Beware: if the input
    language is not co-Büchi realizable the output automaton will
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    recognize a superset of the input.  Currently, the output is
    always state-based.
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  - genltl learned to generate six new families of formulas, taken from
    the SYNTCOMP competition on reactive synthesis, and from from
    Müller & Sickert's GandALF'17 paper:
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    --gf-equiv=RANGE       (GFa1 & GFa2 & ... & GFan) <-> GFz
    --gf-implies=RANGE     (GFa1 & GFa2 & ... & GFan) -> GFz
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    --ms-example=RANGE     GF(a1&X(a2&X(a3&...)))&F(b1&F(b2&F(b3&...)))
    --ms-phi-h=RANGE       FG(a|b)|FG(!a|Xb)|FG(a|XXb)|FG(!a|XXXb)|...
    --ms-phi-r=RANGE       (FGa{n}&GFb{n})|((FGa{n-1}|GFb{n-1})&(...))
    --ms-phi-s=RANGE       (FGa{n}|GFb{n})&((FGa{n-1}&GFb{n-1})|(...))
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  - autfilt learned --streett-like to convert automata with DNF
    acceptance into automata with Streett-like acceptance.
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  - autfilt learned --acceptance-is=ACC to filter automata by
    acceptance condition.  ACC can be the name of some acceptance
    class (e.g. Büchi, Fin-less, Streett-like) or a precise acceptance
    formula in the HOA syntax.

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  - ltldo learned to limit the number of automata it outputs using -n.

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  - ltlfilt learned to measure wall-time using --format=%r, and
    cpu-time with --format=%R.
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  - The --format=%g option of tools that output automata used to
    print the acceptance condition as a *formula* in the HOA format.
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    This %g may now take optional arguments to print the acceptance
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    *name* in different formats.  For instance

       ... | autfilt -o '%[s]g.hoa'

    will separate a stream of automata into different files named
    by their acceptance name (Buchi, co-Buchi, Streett, etc.) or
    "other" if no name is known for the acceptance condition.

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  - Tools that produce formulas now support --format=%[OP]n to
    display the nesting depth of operator OP.

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  - The new -x tls-impl=N option allows to fine-tune the
    implication-based simplification rules of ltl2tgba.  See the
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    spot-x(7) man page for details.
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  - All tools learned to check the SPOT_OOM_ABORT environment
    variable.  This is only useful for debuging out-of-memory
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    conditions. See the spot-x(7) man page for details.
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  New functions in the library:

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  - spot::iar() and spot::iar_maybe() use index appearance records (IAR)
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    to translate Rabin-like automata into equivalent parity automata.
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    This translation preserves determinism and is especially useful when
    the input automaton is deterministic.

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  - spot::print_aiger() encodes an automaton as an AIGER circuit, as
    required by the SYNTCOMP competition.  It relies on a new named
    property "synthesis outputs" that describes which atomic
    propositions are to be encoded as outputs of the circuits.

  - spot::dnf_to_streett() converts any automaton with a DNF
    acceptance condition into a Streett-like automaton.

  - spot::nsa_to_nca(), spot::dfn_to_nca(), spot::dfn_to_dca(), and
    spot::nsa_to_dca(), convert automata with DNF or Streett-like
    acceptance into deterministic or non-deterministic co-Büchi
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    automata.  spot::to_dca() and spot::to_nca() dispatch between
    these four functions.  The language of produced automaton includes
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    the original language, but may be larger if the original automaton
    is not co-Büchi realizable.  Based on Boker & Kupferman FOSSACS'11
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    paper.  Currently only supports state-based output.
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  - spot::scc_info::states_on_acc_cycle_of() returns all states
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    visited by any accepting cycle of the specified SCC.  It only
    works on automata with Streett-like acceptance.

  - spot::is_recurrence(), spot::is_persistence(), and
    spot::is_obligation() test if a formula is a recurrence,
    persistance or obligation.  The SPOT_PR_CHECK and SPOT_O_CHECK
    environment variables can be used to select between multiple
    implementations of these functions (see the spot-x(7) man page).

  - spot::is_colored() checks if an automaton is colored
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    (i.e., every transition belongs to exactly one acceptance set).
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  - spot::change_parity() converts between different parity acceptance
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    conditions.
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  - spot::colorize_parity() transforms an automaton with parity
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    acceptance into a colored automaton (which is often what people
    working with parity automata expect).
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  - spot::cleanup_parity_acceptance() simplifies a parity acceptance
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    condition.
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  - spot::parity_product() and spot::parity_product_or() are
    specialized (but expensive) products that preserve parity
    acceptance.
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  - spot::remove_univ_otf() removes universal transitions on the fly
    from an alternating Büchi automaton using Miyano and Hayashi's
    breakpoint algorithm.
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  - spot::stutter_invariant_states(),
    spot::stutter_invariant_letters(),
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    spot::highlight_stutter_invariant_states(), ...  These functions
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    help study a stutter-sensitive automaton and detect the subset of
    states that are stutter-invariant.  See
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    https://spot.lrde.epita.fr/ipynb/stutter-inv.html for examples.
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  - spot::acc_cond::name(fmt) is a new method that names well-known
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    acceptance conditions.  The fmt parameter specifies the format to
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    use for that name (e.g. to the style used in HOA, or that used by
    print_dot()).
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  - spot::formula::is_leaf() method can be used to detect formulas
    without children (atomic propositions, or constants).
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  - spot::nesting_depth() computes the nesting depth of any LTL
    operator.
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  - spot::check_determinism() sets both prop_semi_deterministic()
    and prop_universal() appropriately.
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  Improvements to existing functions in the library:

  - spot::tgba_determinize() has been heavily rewritten and
    optimized. The algorithm has (almost) not changed, but it is
    much faster now. It also features an optimization for
    stutter-invariant automata that may produce slightly smaller
    automata.

  - spot::scc_info now takes an optional argument to disable some
    features that are expensive and not always necessary.  By
    default scc_info tracks the list of all states that belong to an
    SCC (you may now ask it not to), tracks the successor SCCs of
    each SCC (that can but turned off), and explores all SCCs of the
    automaton (you may request to stop on the first SCC that is
    found accepting).

  - In some cases, spot::degeneralize() would output Büchi automata
    with more SCCs than its input.  This was hard to notice, because
    very often simulation-based simplifications remove those extra
    SCCs.  This situation is now detected by spot::degeneralized()
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    and fixed before returning the automaton.  A new optional
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    argument can be passed to disable this behavior (or use -x
    degen-remscc=0 from the command-line).

  - The functions for detecting stutter-invariant formulas or
    automata have been overhauled.  Their interface changed
    slightly.  They are now fully documented.

  - spot::postprocessor::set_type() can now request different forms
    of parity acceptance as output.  However currently the
    conversions are not very smart: if the input does not already
    have parity acceptance, it will simply be degeneralized or
    determinized.

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  - spot::postprocessor::set_type() can now request co-Büchi
    acceptance as output.  This calls the aforementioned to_nca() or
    to_dca() functions.

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  - spot::remove_fin() will now call simplify_acceptance(),
    introduced in 2.4, before attempting its different Fin-removal
    strategies.

  - spot::simplify_acceptance() was already merging identical
    acceptance sets, and detecting complementary sets i and j to
    perform the following simplifications
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      Fin(i) & Inf(j) = Fin(i)
      Fin(i) | Inf(j) = Inf(i)
    It now additionally applies the following rules (again assuming i
    and j are complementary):
      Fin(i) & Fin(j) = f          Inf(i) | Inf(j) = t
      Fin(i) & Inf(i) = f          Fin(i) | Inf(i) = t

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  - spot::formula::map(fun) and spot::formula::traverse(fun) will
    accept additionnal arguments and pass them to fun().  See
    https://spot.lrde.epita.fr/tut03.html for some examples.

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  Python-specific changes:
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  - The "product-states" property of automata is now accessible via
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    spot.twa.get_product_states() and spot.twa.set_product_states().
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  - twa_word instances can be displayed as SVG pictures, with one
    signal per atomic proposition.  For some examples, see the use of
    the show() method in https://spot.lrde.epita.fr/ipynb/word.html

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  - The test suite is now using v4 of the Jupyter Notebook format.

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  - The function rabin_is_buchi_realizable() as its name suggests checks
    if a rabin aut. is Büchi realizable. It is heavily based on
    tra_to_tba() algorithm.

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  Deprecation notices:

  (These functions still work but compilers emit warnings.)

  - spot::scc_info::used_acc(), spot::scc_info::used_acc_of() and
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    spot::scc_info::acc() are deprecated.  They have been renamed
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    spot::scc_info::marks(), spot::scc_info::marks_of() and
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    spot::scc_info::acc_sets_of() respectively for clarity.
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