Add an is_unambiguous() function, use it in ltlcross and autfilt

* src/twaalgos/isunamb.hh, src/twaalgos/isunamb.cc: New files.
* src/twaalgos/Makefile.am: Add them.
* src/tests/unambig.test: New file.
* src/tests/Makefile.am: Add it.
* src/bin/ltlcross.cc: Record whether each produced automaton is
ambiguous.
* src/bin/autfilt.cc: Add a --is-unambiguous option.
* NEWS: Mention it.

src/twaalgos/isunamb.hh

@@ -0,0 +1,44 @@
+// -*- coding: utf-8 -*-
+// Copyright (C) 2013, 2015 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 this program.  If not, see <http://www.gnu.org/licenses/>.
+
+#pragma once
+
+#include "twa/twagraph.hh"
+
+namespace spot
+{
+  class tgba;
+
+  /// \addtogroup tgba_misc
+  /// @{
+
+  /// \brief Whether the automaton \a aut is unambiguous.
+  ///
+  /// An automaton is unambiguous if each accepted word is
+  /// recognized by only one path.
+  ///
+  /// We check unambiguousity by synchronizing the automaton with
+  /// itself, and then making sure that the co-reachable part of the
+  /// squared automaton has the same size as the co-reachable part of
+  /// the original automaton.
+  SPOT_API bool
+  is_unambiguous(const const_twa_graph_ptr& aut);
+
+  /// @}
+}