Implement star-normal-form rewriting.

* src/ltlvisit/snf.cc, src/ltlvisit/snf.hh: New files.
* src/ltlvisit/Makefile.am: Distribute them.
* src/ltlvisit/simplify.cc, src/ltlvisit/simplify.hh: Call snf(f) for
all f[*].
* src/ltltest/reduccmp.test: Test it.
* doc/tl/tl.tex, doc/tl/tl.bib: Document it.

src/ltlvisit/snf.hh

@@ -0,0 +1,60 @@
+// -*- coding: utf-8 -*-
+// Copyright (C) 2012 Laboratoire de Recherche et Developpement
+// 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 3 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_SNF_HH
+#define SPOT_LTLVISIT_SNF_HH
+
+#include "ltlast/formula.hh"
+#include "misc/hash.hh"
+
+namespace spot
+{
+  namespace ltl
+  {
+
+    typedef Sgi::hash_map<const formula*, const formula*,
+			  ptr_hash<formula> > snf_cache;
+
+    /// Helper to rewrite a sere in Star Normal Form.
+    ///
+    /// This should only be called on children of a Star operator.  It
+    /// corresponds to the E° operation defined in the following
+    /// paper.
+    ///
+    /// \verbatim
+    /// @Article{	  bruggeman.96.tcs,
+    ///   author	= {Anne Br{\"u}ggemann-Klein},
+    ///   title		= {Regular Expressions into Finite Automata},
+    ///   journal	= {Theoretical Computer Science},
+    ///   year		= {1996},
+    ///   volume	= {120},
+    ///   pages		= {87--98}
+    /// }
+    /// \endverbatim
+    ///
+    /// \param sere the SERE to rewrite
+    /// \param cache an optional cache
+    formula* star_normal_form(const formula* sere,
+			      snf_cache* cache = 0);
+  }
+}
+
+#endif // SPOT_LTLVISIT_SNF_HH