not a formula&.  This is more homogeneous.
* src/ltlvisit/dump.cc (dump): Likewise.
* src/ltlvisit/dotty.hh (dotty): Likewise.
* src/ltlvisit/dotty.cc (dotty): Likewise.
* src/ltlvisit/tostring.hh (to_string): Likewise.
* src/ltlvisit/tostring.cc (to_string): Likewise.
* src/ltltest/readltl.cc, src/ltltest/equals.cc,
src/ltltest/tostring.cc: Adjust usage.