Add support for {SERE} and !{SERE} closure operators.

* src/ltlast/unop.hh, src/ltlast/unop.cc: Introduce Closure and
NegClosure operators.
* src/ltlparse/ltlparse.yy: Recognize {foo} as a Closure.
* src/ltlvisit/mark.cc: Consider NegClosure as a marked operator.
* src/tgbaalgos/ltl2tgba_fm.cc (ratexp_trad_visitor): Add option to
select whether the empty_word should act like true (for {SERE}
and {!SERE}) or false (for {SERE}<>->Exp or {SERE}[]->Exp).
(ltl_trad_visitor): Translate Closure and NegClosure.
* src/tgbatest/ltl2tgba.test: Add more tests.
* src/ltlvisit/basicreduce.cc, src/ltlvisit/consterm.cc,
src/ltlvisit/nenoform.cc, src/ltlvisit/reduce.cc,
src/ltlvisit/syntimpl.cc, src/ltlvisit/tostring.cc,
src/ltlvisit/tunabbrev.cc, src/tgba/formula2bdd.cc,
src/tgbaalgos/eltl2tgba_lacim.cc, src/tgbaalgos/ltl2tgba_lacim.cc,
src/tgbaalgos/ltl2taa.cc: Straightforward update to support or
assert on these new operators.

src/ltlparse/ltlparse.yy

@@ -361,6 +361,8 @@ subformula: booleanatom
 	      { $$ = unop::instance(unop::Not, $2); }
 	    | OP_NOT error
 	      { missing_right_op($$, @1, "not operator"); }
+            | bracedrationalexp
+	      { $$ = unop::instance(unop::Closure, $1); }
             | bracedrationalexp OP_UCONCAT subformula
 	      { $$ = binop::instance(binop::UConcat, $1, $3); }
             | bracedrationalexp OP_UCONCAT error
@@ -369,6 +371,7 @@ subformula: booleanatom
 	      { $$ = binop::instance(binop::EConcat, $1, $3); }
             | bracedrationalexp OP_ECONCAT error
 	      { missing_right_binop($$, $1, @2, "universal concat operator"); }
+
 ;
 
 %%