to_string: abbreviate [->i..j] and [=i..j] expressed using [*i..j]

* src/ltlast/bunop.hh (is_bunop, is_Star): New functions.
* src/ltlast/multop.hh (is_And, is_Or): Fix constness.
(is_Concat, is_Fusion): New functions.
* src/ltlast/unop.hh (is_unop, is_X, is_F, is_G, is_GF, is_FG):
Fix constness.
(is_Not): New function.
* src/ltlvisit/tostring.cc (strip_star_not, match_goto,
emit_bunop_child, resugar_concat): New methods.
(visit(bunop)): Rewrite without calling format().  Detect the
[->i..j] pattern.
(visit(multop)): Call resugar_concat to detect [=i..j] patterns.

src/ltlast/bunop.hh

@@ -1,4 +1,4 @@
-// Copyright (C) 2010, 2011 Laboratoire de Recherche et Développement
+// Copyright (C) 2010, 2011, 2012 Laboratoire de Recherche et Développement
 // de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -120,6 +120,45 @@ namespace spot
       unsigned max_;
     };
 
+    /// \brief Cast \a f into a bunop.
+    ///
+    /// Cast \a f into a bunop iff it is a bunop instance.  Return 0
+    /// otherwise.  This is faster than \c dynamic_cast.
+    inline
+    bunop*
+    is_bunop(const formula* f)
+    {
+      if (f->kind() != formula::BUnOp)
+	return 0;
+      return static_cast<bunop*>(const_cast<formula*>(f));
+    }
+
+    /// \brief Cast \a f into a bunop if it has type \a op.
+    ///
+    /// Cast \a f into a bunop iff it is a bunop instance with operator \a op.
+    /// Returns 0 otherwise.
+    inline
+    bunop*
+    is_bunop(const formula* f, bunop::type op)
+    {
+      if (f->kind() != formula::BUnOp)
+	return 0;
+      bunop* bo = static_cast<bunop*>(const_cast<formula*>(f));
+      if (bo->op() != op)
+	return 0;
+      return bo;
+    }
+
+    /// \brief Cast \a f into a bunop if it is a Star.
+    ///
+    /// Return 0 otherwise.
+    inline
+    bunop*
+    is_Star(const formula* f)
+    {
+      return is_bunop(f, bunop::Star);
+    }
+
   }
 }
 #endif // SPOT_LTLAST_BUNOP_HH