Implement W,M removal for Spin output.

* src/ltlvisit/wmunabbrev.hh, src/ltlvisit/wmunabbrev.cc: New files.
* src/ltlvisit/Makefile.am: Add them.
* src/ltlvisit/tostring.cc (to_spin_string): Use the new rewriting.
* wrap/python/ajax/spot.in: Warn when a "Spin" still contain PSL
operators.
* wrap/python/ajax/ltl2tgba.html: Adjust help text.
* doc/tl/tl.tex, NEWS: Document the new rewriting.

src/ltlvisit/wmunabbrev.hh

@@ -0,0 +1,44 @@
+// -*- coding: utf-8 -*-
+// Copyright (C) 2012 Laboratoire de Recherche et Développement
+// de l'Epita (LRDE).
+//
+// This file is part of Spot, a model checking library.
+//
+// Spot is free software; you can redistribute it and/or modify it
+// under the terms of the GNU General Public License as published by
+// the Free Software Foundation; either version 2 of the License, or
+// (at your option) any later version.
+//
+// Spot is distributed in the hope that it will be useful, but WITHOUT
+// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+// or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+// License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Spot; see the file COPYING.  If not, write to the Free
+// Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
+// 02111-1307, USA.
+
+#ifndef SPOT_LTLVISIT_WMUNABBREV_HH
+# define SPOT_LTLVISIT_WMUNABBREV_HH
+
+namespace spot
+{
+  namespace ltl
+  {
+    class formula;
+
+    /// \brief Rewrite a formula to remove the W and M operators.
+    ///
+    /// This is necessary if you want to use the formula with a tool
+    /// that do not support these operators.
+    ///
+    /// <code>a W b</code> is replaced by <code>b R (b | a)</code>,
+    /// and <code>a M b</code> is replaced by <code>b U (b & a)</code>.
+    ///
+    /// \ingroup ltl_rewriting
+    formula* unabbreviate_wm(const formula* f);
+  }
+}
+
+#endif // SPOT_LTLVISIT_WMUNABBREV_HH