ltlcross: adding option --grind=FILENAME

Suggested by Joachim Klein.

When a bogus formula is found by ltlcross, the --grind=FILENAME option
tries to find a smaller formula for which the bug is still present, and
outputs it in FILENAME.

* src/bin/ltlcross.cc: Add the --grind option.
* doc/org/ltlcross.org: Document the --grind option.
* src/ltltest/ltlcrossgrind.test: Test it.
* src/ltltest/Makefile.am: Add test.

src/ltltest/Makefile.am

@@ -99,6 +99,7 @@ TESTS = \
   remove_x.test \
   ltlrel.test \
   ltlgrind.test \
+  ltlcrossgrind.test \
   ltlfilt.test \
   latex.test \
   lbt.test \

src/ltltest/ltlcrossgrind.test

@@ -0,0 +1,35 @@
+#! /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 || exit 1
+
+set -e
+
+cat >fake <<EOF
+#!/bin/sh
+../../bin/ltl2tgba -s -f "\$1" | sed 's/p0/p1/g'
+EOF
+
+chmod +x fake
+
+run 1 ../../bin/ltlcross -f 'p0 U p1' "./fake %f >%N" \
+"../../bin/ltl2tgba -s -f %f >%N" --grind=bogus.grind
+
+echo p0 >exp
+diff bogus.grind exp