* src/ltlast/multop.cc, src/ltlast/multop.hh: Declare AndNML
operator.
* src/ltlparse/ltlscan.ll: Distinguish "&" and "&&".
* src/ltlparse/ltlparse.yy: Handle them both as "And" for LTL
formula, use AndNML or And for rational expressions.
* src/ltlvisit/tostring.cc: Adjust to distinguish "&" and "&&" in
rational expressions. Also use {braces} to group rational
expressions.
* src/tgbaalgos/ltl2tgba_fm.cc
(ratexp_trad_visitor::ratexp_trad_visitor): Remove the possibility
to select the empty_word should act like true, and fix the rules
for Closure and NegClosure to rely on constant_term instead.
(ratexp_trad_visitor::visit) Adjust the And translation to also
support AndNML.
(ratexp_trad_visitor::recurse_and_concat): Introduce this new
method to simplify some calls to recurse(f, to_concat_).
* src/tgbatest/ltl2tgba.test: Add more test cases.
* src/ltlvisit/basicreduce.cc, src/ltlvisit/consterm.cc,
src/ltlvisit/contain.cc, src/ltlvisit/mark.cc,
src/ltlvisit/nenoform.cc, src/ltlvisit/syntimpl.cc,
src/tgba/formula2bdd.cc, src/tgbaalgos/eltl2tgba_lacim.cc,
src/tgbaalgos/ltl2taa.cc, src/tgbaalgos/ltl2tgba_lacim.cc: Add
missing cases in switches.