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.
(multop::add): If the formulae we add is itself a multop for the
same operator, merge its children with ours.
* src/ltltest/parseerr.test: Add two tests for multop merging.
