Use [*0] instead of #e, and support [*] in addition to *.

* src/ltlparse/ltlscan.ll: Recognize [*] as *, and use
[*0] instead of #e for the empty word.
* src/ltlast/binop.cc, src/ltlast/constant.cc,
src/ltlast/multop.cc, src/ltlast/multop.hh, src/ltlast/unop.cc,
src/ltlast/unop.hh, src/ltltest/consterm.test,
src/ltltest/equals.test: Adjust all occurrences of #e to [*0].
* src/tgbatest/ltl2tgba.test: Also use [*].

src/ltlast/binop.cc

@@ -261,7 +261,7 @@ namespace spot
 	case EConcatMarked:
 	  //   - 0 <>-> Exp = 0
 	  //   - 1 <>-> Exp = Exp
-	  //   - #e <>-> Exp = 0
+	  //   - [*0] <>-> Exp = 0
 	  //   - Exp <>-> 0 = 0
 	  if (first == constant::true_instance())
 	    return second;
@@ -280,7 +280,7 @@ namespace spot
 	case UConcat:
 	  //   - 0 []-> Exp = 1
 	  //   - 1 []-> Exp = Exp
-	  //   - #e []-> Exp = 1
+	  //   - [*0] []-> Exp = 1
 	  //   - Exp []-> 1 = 1
 	  if (first == constant::true_instance())
 	    return second;

src/ltlast/constant.cc

@@ -87,7 +87,7 @@ namespace spot
 	case False:
 	  return "0";
 	case EmptyWord:
-	  return "#e";
+	  return "[*0]";
 	}
       // Unreachable code.
       assert(0);

src/ltlast/multop.cc

@@ -239,9 +239,9 @@ namespace spot
 		++i;
 	      }
 	  }
-	// We have    a* & #e & 0  = 0  // already checked above
-	//     but    a* & #e & c*  = #e
-	// So if #e has been seen, check if all term recognize the
+	// We have    a* & [*0] & 0  = 0  // already checked above
+	//     but    a* & [*0] & c* = [*0]
+	// So if [*0] has been seen, check if all term recognize the
 	// empty word.
 	if (weak_abs_seen)
 	  {

src/ltlast/multop.hh

@@ -80,7 +80,7 @@ namespace spot
       /// are also taken care of.  The following rewriting are performed
       /// (the left patterns are rewritten as shown on the right):
       ///
-      /// - Concat(Exps1...,#e,Exps2...) = Concat(Exps1...,Exps2...)
+      /// - Concat(Exps1...,[*0],Exps2...) = Concat(Exps1...,Exps2...)
       /// - Concat(Exps1...,0,Exps2...) = 0
       /// - Concat(Exp) = Exp
       /// - And(Exps1...,1,Exps2...) = And(Exps1...,Exps2...)

src/ltlast/unop.cc

@@ -120,8 +120,8 @@ namespace spot
       // Some trivial simplifications.
       switch (op)
 	{
-	  // We have (0*) == (#e)
-	  //         (#e*) == (#e)
+	  // We have (0*) == ([*0])
+	  //         ([*0]*) == ([*0])
 	case Star:
 	  if (child == constant::false_instance()
 	      || child == constant::empty_word_instance())
@@ -145,7 +145,7 @@ namespace spot
 	    if (child == constant::false_instance()
 		|| child == constant::true_instance())
 	      return child;
-	    // F(#e) = G(#e) = 1
+	    // F([*0]) = G([*0]) = 1
 	    if (child == constant::empty_word_instance())
 	      return constant::true_instance();
 	  }
@@ -192,7 +192,7 @@ namespace spot
 	  if (child == constant::true_instance()
 	      || child == constant::false_instance())
 	    return child;
-	  // X(#e) = 1
+	  // X([*0]) = 1
 	  if (child == constant::empty_word_instance())
 	    return constant::true_instance();
 	  break;

src/ltlast/unop.hh

@@ -57,8 +57,8 @@ namespace spot
       /// The following trivial simplifications are performed
       /// automatically (the left expression is rewritten as the right
       /// expression):
-      ///   - 0* = #e
-      ///   - #e* = #e
+      ///   - 0* = [*0]
+      ///   - [*0]* = [*0]
       ///   - Exp** = Exp*
       ///   - FF(Exp) = F(Exp)
       ///   - GG(Exp) = G(Exp)
@@ -66,17 +66,17 @@ namespace spot
       ///   - G(0) = 0
       ///   - F(1) = 1
       ///   - G(1) = 1
-      ///   - F(#e) = 1
-      ///   - G(#e) = 1
+      ///   - F([*0]) = 1
+      ///   - G([*0]) = 1
       ///   - !1 = 0
       ///   - !0 = 1
       ///   - !!Exp = Exp
       ///   - !Closure(Exp) = NegClosure(Exp)
       ///   - !NegClosure(Exp) = Closure(Exp)
-      ///   - Closure(#e) = 1
+      ///   - Closure([*0]) = 1
       ///   - Closure(1) = 1
       ///   - Closure(0) = 0
-      ///   - NegClosure(#e) = 0
+      ///   - NegClosure([*0]) = 0
       ///   - NegClosure(1) = 0
       ///   - NegClosure(0) = 1
       ///