Rewrite Exp[=0..] as [*].

* src/ltlast/bunop.cc: Implement this rewriting.
* src/ltlast/bunop.hh: Document it.
* src/ltltest/equals.test: Test it.

src/ltlast/bunop.cc

@@ -167,13 +167,22 @@ namespace spot
 	{
 	case Equal:
 	  {
+	    //   - Exp[=0..] = [*]
+	    if (min == 0 && max == unbounded)
+	      {
+		op = Star;
+		child->destroy();
+		child = constant::true_instance();
+		break;
+	      }
+
 	    //   - 0[=0..max] = [*]
 	    //   - 0[=min..max] = 0 if min > 0
 	    if (child == constant::false_instance())
 	      {
 		if (min == 0)
 		  {
-		    max = -1U;
+		    max = unbounded;
 		    op = Star;
 		    child = constant::true_instance();
 		    break;

src/ltlast/bunop.hh

@@ -55,6 +55,7 @@ namespace spot
       ///   - 0[=min..max] = 0 if min > 0
       ///   - 1[=0] = [*0]
       ///   - 1[=min..max] = 1[*min..max] if max > 0
+      ///   - Exp[=0..] = [*]
       ///   - Exp[=0] = (!Exp)[*]
       ///
       /// These rewriting rules imply that it is not possible to build

src/ltltest/equals.test

@@ -150,3 +150,4 @@ run 0 ../equals '{1[=0]}' '{[*0]}'
 run 0 ../equals '{1[=1..2]}' '{[*1..2]}'
 run 0 ../equals '{1[=..4]}' '{1[*..4]}'
 run 0 ../equals '{b[=0]}' '{(!b)[*]}'
+run 0 ../equals '{b[=0..]}' '{*}'