Introduce [=min..max] operator.

* src/ltlast/bunop.hh: Declare bunop::Equal
* 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::Equal in
the translation.
* src/ltltest/equals.test: Test trivial identities
for [=min..max].
* src/tgbatest/ltl2tgba.test: Add new formulae to test.

src/ltlast/bunop.hh

@@ -37,7 +37,7 @@ namespace spot
     class bunop : public ref_formula
     {
     public:
-      enum type { Star };
+      enum type { Star, Equal };
 
       static const unsigned unbounded = -1U;
 
@@ -49,7 +49,13 @@ namespace spot
       ///   - 0[*0..max] = [*0]
       ///   - 0[*min..max] = 0 if min > 0
       ///   - [*0][*min..max] = [*0]
+      ///   - Exp[*0] = [*0]
       ///   - Exp[*i..j][*k..l] = Exp[*ik..jl] if i*(k+1)<=jk+1.
+      ///   - 0[=0..max] = 1[*]
+      ///   - 0[=min..max] = 0 if min > 0
+      ///   - 1[=0] = [*0]
+      ///   - 1[=min..max] = 1[*min..max] if max > 0
+      ///   - Exp[=0] = (!Exp)[*]
       ///
       /// These rewriting rules imply that it is not possible to build
       /// an LTL formula object that is SYNTACTICALLY equal to one of