* src/tgbaalgos/ltl2tgba_fm.cc (ratexp_to_dfa::translate): Do not
translate a subformula if we have already proved it useless in
a previous rational expression.
* src/tgbatest/ltl2tgba.test: Add an example, although that
test does not ensure the subformula is ignored early in the
translation.  I.e., it would still work without the patch.