* src/ltlvisit/tostring.hh (to_string): Add doxygen comments.
* src/ltlast/multop.hh (multop::paircmp): Add doxygen comments.
* src/ltlvisit/postfix.hh: Typo.

reference-counted formula*.  One can't call constructors for AST
items anymore, everything need to be acquired through instance()
class methods.

* src/ltlast/formula.cc, src/ltlast/refformula.cc,
src/ltlast/refformula.hh: New files.
* src/ltlast/Makefile.am (libltlast_la_SOURCES): Add them.
* src/ltlast/atomic_prop.cc, src/ltlast/atomic_prop.hh,
src/ltlast/unop.cc, src/ltlast/unop.hh,
src/ltlast/binop.cc, src/ltlast/binop.hh: Make the constructor
and destructor protected.  Define a static function `instance()'
to get an instance with specific argument.  Use a map called
`instances' to store all known instances.  Inherit from
ref_formula.
* src/ltlast/constant.hh, src/ltlast/constant.cc: Protect
the constructor and destructor.  Provide the false_instance()
and true_instance() functions instead.
* src/formula.hh (ref, unref, ref_, unref_): New methods.
* src/ltlast/multop.cc, src/ltlast/multop.hh: Protect
the constructor, destructor, as well as the add() method.
Provides the instance(), and add() class methods instead.
Store children_ as a pointer.
* src/ltlenv/defaultenv.cc (require): Adjust to
call atomic_prop::instance.
* src/ltlparse/ltlparse.yy: Adjust to call instance() functions
instead of constructors.
* src/ltltest/Makefile.am (LDADD): Tweak library ordering.
* src/ltlvisit/clone.hh (clone_visitor): Inherit from visitor,
not const_visitor, and adjust all prototypes appropriately.
* src/ltlvisit/clone.cc (clone_visitor): Likewise.
Call ref() or instance() methods instead of copy constructors.
* src/ltlvisit/equals.cc: Simplify atomic_prop and constant
cases.
* src/ltlvisit/lunabbrev.hh, src/ltlvisit/lunabbrev.cc,
src/ltlvisit/tunabbrev.hh, src/ltlvisit/tunabbrev.cc,
src/ltlvisit/nenoform.hh, src/ltlvisit/nenoform.cc: Use instance()
methods instead of constructor.  Make these children of visitor, not
const_visitor.
* src/ltltest/readltl.c (main): Do not delete the formula.

* src/ltlast/Makefile.am, src/ltlenv/Makefile.am,
src/ltlparse/Makefile.am, src/ltltest/Makefile.am,
src/ltlvisit/Makefile.am: Adust to build libtool libraries.

* src/ltltest/tostring.cc: New files.
* src/ltlvisit/tostring.hh: From ast to string New files.
* src/ltlvisit/tostring.cc: From ast to string New files.

* src/ltlenv/Makefile.am, src/ltlenv/defaultenv.cc,
src/ltlenv/defaultenv.hh, src/ltlenv/environment.hh: New files.
* src/ltlparse/public.hh (parse): Take an environment as third
argument.
* src/ltlparse/ltlparse.yy (ATOMIC_PROP, parse): Require the
atomic proposition via the environment.
* src/ltltest/readltl.cc (main): Adjust the call to parse().
* src/ltltest/Makefile.am (LDADD): Add ../ltlenv/libltlenv.a.

* src/ltlvisit/Makefile.am (libltlvisit_a_SOURCES): Add them.
* src/ltltest/equals.cc (main) [NENOFORM]: Call negative_normal_form.
* src/ltltest/nenoform.test, src/ltltest/tunenoform.test: New files.
* src/ltltest/Makefile.am (check_PROGRAMS): Add nenoform and
tunenoform.
(nenoform_SOURCES, nenoform_CPPFLAGS, tunenoform_SOURCES,
tunenoform_CPPFLAGS): New variables.
(TESTS): Add nenoform.test and tunenoform.test.

* src/ltlvisit/Makefile.am (libltlvisit_a_SOURCES): Add them.
* src/ltltest/tunabbrev.test: New file.
* src/ltltest/lunabbrev.test: Fix comment.
* src/ltltest/Makefile.am (TESTS): Add tunabbrev.test.
(check_PROGRAMS): Add tunabbrev.
(tunabbrev_SOURCES, tunabbrev_CPPFLAGS): New variables.
* src/ltltest/equals.cc (main) [TUNABBREV]: Call unabbreviate_ltl.
* src/ltlvisit/lunabbrev.hh (unabbreviate_logic_visitor::recurse):
New virtual function.
* src/ltlvisit/lunabbrev.cc (unabbreviate_logic_visitor::recurse):
Likewise.
(unabbreviate_logic_visitor::visit): Use it instead of calling
unabbreviate_logic directly.

* src/ltlvisit/Makefile.am (libltlvisit_a_SOURCES): Add them.
* src/ltlast/multop.cc (multop::multop(type)): New constructor.
* src/ltlast/multop.hh (multop::multop(type)): New constructor.
* src/ltltest/lunabbrev.test: New file.
* src/ltltest/Makefile.am (TESTS): Add lunabbrev.test.
(check_PROGRAMS): Add lunabbrev.
(lunabbrev_SOURCES, lunabbrev_CPPFLAGS): New variables.
* src/ltltest/equals.cc (main) [LUNABBREV]: Call unabbreviate_logic.

* src/ltlvisit/Makefile.am (libltlvisit_a_SOURCES): Add equals.hh
and equals.cc.
* src/ltltest/equals.cc, src/ltltest/equals.test: New files.
* src/ltltest/Makefile.am (check_PROGRAMS): Add equals.
(equals_SOURCES): New variable.
(TESTS): Add equals.test.

src/Makefile.am, src/ltlast/Makefile.am, src/ltlast/allnodes.hh,
src/ltlast/atomic_prop.cc, src/ltlast/atomic_prop.hh,
src/ltlast/binop.cc, src/ltlast/binop.hh, src/ltlast/constant.cc,
src/ltlast/constant.hh, src/ltlast/formulae.hh,
src/ltlast/multop.cc, src/ltlast/multop.hh, src/ltlast/predecl.hh,
src/ltlast/unop.cc, src/ltlast/unop.hh, src/ltlast/visitor.hh,
src/ltlparse/Makefile.am, src/ltlparse/ltlparse.yy,
src/ltlparse/ltlscan.ll, src/ltlparse/parsedecl.hh,
src/ltlparse/public.hh, src/ltlvisit/Makefile.am,
src/ltlvisit/dotty.cc, src/ltlvisit/dotty.hh,
src/ltlvisit/dump.cc, src/ltlvisit/dump.hh,
src/ltlvisit/rewrite.cc, src/ltlvisit/rewrite.hh,
src/ltltest/Makefile.am, src/ltltest/defs.in, src/ltltest/readltl.cc,
src/ltltest/parse.test, src/ltltest/parseerr.test,
src/misc/Makefile.am, src/misc/const_sel.hh: New files.