from_ltlf: new LTL transformation.

Fixes #187.

* spot/tl/ltlf.cc, spot/tl/ltlf.hh: New files.
* spot/tl/Makefile.am: Add them.
* bin/ltlfilt.cc: Add a new option.
* bin/man/ltlfilt.x: Add bibliographic reference.
* tests/core/ltlfilt.test: Add more tests.
* tests/python/ltlf.py: New file.
* tests/Makefile.am: Add it.
* python/spot/impl.i: Python bindings.
* NEWS: Mention it.

tests/python/ltlf.py

@@ -0,0 +1,47 @@
+# -*- mode: python; coding: utf-8 -*-
+# Copyright (C) 2016 Laboratoire de Recherche et Développement de
+# l'Epita
+#
+# 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/>.
+
+import spot
+
+lcc = spot.language_containment_checker()
+
+formulas = ['GFa', 'FGa', '(GFa) U b',
+            '(a U b) U c', 'a U (b U c)',
+            '(a W b) W c', 'a W (b W c)',
+            '(a R b) R c', 'a R (b R c)',
+            '(a M b) M c', 'a M (b M c)',
+            '(a R b) U c', 'a U (b R c)',
+            '(a M b) W c', 'a W (b M c)',
+            '(a U b) R c', 'a R (b U c)',
+            '(a W b) M c', 'a M (b W c)',
+           ]
+
+# The rewriting assume the atomic proposition will not change
+# once we reache the non-alive part.
+cst = spot.formula('G(X!alive => ((a <=> Xa) && (b <=> Xb) && (c <=> Xc)))')
+
+for f in formulas:
+    f1 = spot.formula(f)
+    f2 = f1.unabbreviate()
+    f3 = spot.formula_And([spot.from_ltlf(f1), cst])
+    f4 = spot.formula_And([spot.from_ltlf(f2), cst])
+    print("{}\t=>\t{}".format(f1, f3))
+    print("{}\t=>\t{}".format(f2, f4))
+    assert lcc.equal(f3, f4)
+    print()