* src/bin/ltlfilt.cc: Implement it.
* src/bin/common_output.cc, src/bin/common_output.hh: export the
stream_formula function.
* src/ltltest/ltlfilt.test: Test it.
* src/ltlvisit/relabel.hh: Make it possible to clear the
relabeling map.
* NEWS, doc/org/ltlfilt.org: Mention --define.