src/ltlast/binop.hh, src/ltlast/constant.hh,
src/ltlast/formula.hh, src/ltlast/multop.hh,
src/ltlast/predecl.hh, src/ltlast/refformula.hh,
src/ltlast/unop.hh, src/ltlast/visitor.hh: Use file to introduce
each file.