* 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.
(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.