Introduce [->min..max] operator.

* src/ltlast/bunop.hh: Declare bunop::Goto
* src/ltlast/bunop.cc: Handle it.
* src/ltlparse/ltlparse.yy,
src/ltlparse/ltlscan.ll: Add rules for [->min..max].
* src/tgbaalgos/ltl2tgba_fm.cc: Handle bunop::Goto in
the translation.
* src/ltltest/equals.test: Test trivial identities.
* src/tgbatest/ltl2tgba.test: Test two more formulae using [->].

src/ltlast/bunop.hh

@@ -37,7 +37,7 @@ namespace spot
     class bunop : public ref_formula
     {
     public:
-      enum type { Star, Equal };
+      enum type { Star, Equal, Goto };
 
       static const unsigned unbounded = -1U;
 
@@ -57,6 +57,11 @@ namespace spot
       ///   - 1[=min..max] = 1[*min..max] if max > 0
       ///   - Exp[=0..] = [*]
       ///   - Exp[=0] = (!Exp)[*]
+      ///   - 0[->min..max] = 0 if min>0
+      ///   - 0[->0..max] = [*0]
+      ///   - 1[->0] = [*0]
+      ///   - 1[->min..max] = 1[*min..max]
+      ///   - Exp[->0] = [*0]
       ///
       /// These rewriting rules imply that it is not possible to build
       /// an LTL formula object that is SYNTACTICALLY equal to one of