Make it possible to clone const formulae.

* src/ltlast/formula.hh, src/ltlast/formula.cc (clone): Declare
as const.

ChangeLog

+2009-11-08  Alexandre Duret-Lutz  <adl@lrde.epita.fr>
+
+	Make it possible to clone const formulae.
+
+	* src/ltlast/formula.hh, src/ltlast/formula.cc (clone): Declare
+	as const.
+
 2009-11-08  Alexandre Duret-Lutz  <adl@lrde.epita.fr>
 
 	Rename formula::ref and formula::unref as formula::clone

src/ltlast/formula.cc

@@ -27,10 +27,10 @@ namespace spot
   namespace ltl
   {
     formula*
-    formula::clone()
+    formula::clone() const
     {
-      ref_();
-      return this;
+      const_cast<formula*>(this)->ref_();
+      return const_cast<formula*>(this);
     }
 
     formula::~formula()

src/ltlast/formula.hh

@@ -80,7 +80,7 @@ namespace spot
       ///
       /// This increments the reference counter of this node (if one is
       /// used).
-      formula* clone();
+      formula* clone() const;
       /// \brief release this node
       ///
       /// This decrements the reference counter of this node (if one is