* src/ltlvisit/mark.hh, src/ltlvisit/mark.cc (mark_concat_ops,
simplify_mark): Rewrite these two functions as methods of
(mark_tools): this new class.
* src/ltlast/binop.cc, src/ltlast/unop.cc: Adjust computation
of not_marked to ignore marked operators that are not at
the top-level.  I.e., something like X(!{a}) is not marked.
* src/tgbaalgos/ltl2tgba_fm.cc (translate_dict::mt): New
instance of mark_tools.
(formula_canonizer::translate) Adjust calls to
mark_concat_ops() and simplify_mark().