remove_x: Implement detection of stutter-invariant LTL formulas.

* src/bin/ltlfilt.cc: Add options --remove-x and --stutter-invariant.
* src/ltlvisit/remove_x.cc, src/ltlvisit/remove_x.hh: New files.
* src/ltlvisit/Makefile.am: Add them.
* src/ltltest/remove_x.test: New file.
* src/ltltest/Makefile.am: Add it.
* NEWS: Mention the new algorithms.

src/ltltest/Makefile.am

@@ -90,6 +90,7 @@ TESTS = \
   unabbrevwm.test \
   consterm.test \
   kind.test \
+  remove_x.test \
   lbt.test \
   lenient.test \
   isop.test \

src/ltltest/remove_x.test

@@ -0,0 +1,29 @@
+#! /bin/sh
+# Copyright (C) 2013 Laboratoire de Recherche et Developement to
+# 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/>.
+
+. ./defs
+
+run 0 ../../bin/ltlfilt --remove-x -f 'Xa' --equivalent-to 'Ga | (!a & Fa)'
+
+run 1 ../../bin/ltlfilt --stutter-invariant -f 'Xa'
+
+run 0 ../../bin/ltlfilt --stutter-invariant -f 'F(!a & Xa & Xb)'
+
+run 1 ../../bin/ltlfilt --stutter-invariant -f 'F(Xa & Xb)'
+