src/ltlast/constant.hh, src/ltlast/formula.hh,
src/ltlast/multop.hh, src/ltlast/unop.hh, src/ltlast/visitor.hh,
src/ltlenv/defaultenv.hh, src/ltlenv/environment.hh,
src/ltlparse/public.hh, src/ltlvisit/clone.hh,
src/ltlvisit/dotty.hh, src/ltlvisit/dump.hh,
src/ltlvisit/equals.hh, src/ltlvisit/lunabbrev.hh,
src/ltlvisit/nenoform.hh, src/ltlvisit/tunabbrev.hh: Add
Doxygen comments.
* src/visitor.hh: Do not use const_sel.  This clarify
the code and helps Doxygen.

an atomic_prop.
* src/ltlast/atomic_prop.hh (atomic_prop): New argument env.
(environment_): New member.
(env): New method.
* src/ltlast/atomic_prop.cc (atomic_prop, env): Likewise.
* src/ltlenv/defaultenv.cc (require): Pass *this as the
environment argument to atomic_prop.
* src/ltlvisit/clone.cc (visit(const atomic_prop*)): Also copy
the environment.
* src/ltlvisit/nenoform.cc (visit(const atomic_prop*)): Likewise.

* 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.