* src/ltlenv/defaultenv.hh, src/ltlenv/defaultenv.cc (require): Return
an atomic_prop*, not a formula*.
* src/bin/randaut.cc, src/bin/randltl.cc, src/ltlvisit/apcollect.cc,
src/tgbatest/ltl2tgba.cc, src/tgbatest/randtgba.cc: Do not cast
the return of require().