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.
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