Revert everything related to Damien's work in 2008 (he will commit a new version soon).

Here are the reverted patches:
8c0d1003b0,
25a3114287,
9afbaf6342,
dc0005f4e1,
543190f2bc.

src/ltlast/formula.cc

@@ -0,0 +1,74 @@
+// Copyright (C) 2003, 2005  Laboratoire d'Informatique de Paris 6 (LIP6),
+// département Systèmes Répartis Coopératifs (SRC), Université Pierre
+// et Marie Curie.
+//
+// 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.
+
+#include "formula.hh"
+#include "misc/hash.hh"
+
+namespace spot
+{
+  namespace ltl
+  {
+    formula*
+    formula::ref()
+    {
+      ref_();
+      return this;
+    }
+
+    formula::~formula()
+    {
+    }
+
+    void
+    formula::unref(formula* f)
+    {
+      if (f->unref_())
+	delete f;
+    }
+
+    void
+    formula::ref_()
+    {
+      // Not reference counted by default.
+    }
+
+    bool
+    formula::unref_()
+    {
+      // Not reference counted by default.
+      return false;
+    }
+
+    const std::string&
+    formula::dump() const
+    {
+      return dump_;
+    }
+
+    void
+    formula::set_key_()
+    {
+      string_hash sh;
+      hash_key_ = sh(dump_);
+    }
+
+  }
+}