simulation: do not purge unreachable states when recording implications

This fixes the incorrect output of tgba_determinize() reported yesterday
by Reuben Rowe.

* spot/twaalgos/simulation.cc: Do not purge unreachable states when
recording implications.
* tests/python/bugdet.py: New test case.
* tests/Makefile.am: Add it.
* THANKS: Add Reuben.
* NEWS: Mention the bug.

tests/python/bugdet.py

@@ -0,0 +1,91 @@
+#!/usr/bin/python3
+# -*- 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/>.
+
+# Test case reduced from a report by Reuben Rowe <r.rowe@ucl.ac.uk>
+# sent to the Spot mailing list on 2016-10-31.
+
+import spot
+
+a = spot.automaton("""
+HOA: v1
+States: 5
+Start: 4
+AP: 3 "p_0" "p_1" "p_2"
+Acceptance: 1 Inf(0)
+--BODY--
+State: 0
+[0&!1&2] 1
+[!0&!1&2] 2
+State: 1 {0}
+[0&!1&2] 1
+State: 2 {0}
+[!0&!1&2] 2
+State: 3 {0}
+[0&1&!2] 0
+[!0&1&!2] 3
+State: 4
+[!0&1&!2] 3
+--END--
+""")
+
+b = spot.automaton("""
+HOA: v1
+States: 8
+Start: 0
+AP: 3 "p_0" "p_1" "p_2"
+Acceptance: 1 Inf(0)
+--BODY--
+State: 0
+[t] 0
+[!0&!1&2] 1
+[!0&1&!2] 4
+[!0&1&!2] 5
+[!0&1&!2] 6
+State: 1 {0}
+[!0&!1&2] 1
+State: 2
+[0&!1&2] 7
+State: 3
+[!0&!1&2] 1
+State: 4
+[0&1&!2] 2
+State: 5
+[0&1&!2] 3
+State: 6 {0}
+[!0&1&!2] 6
+State: 7 {0}
+[0&!1&2] 7
+--END--
+""");
+
+# In Reuben's report this first block built an incorrect deterministic
+# automaton, which ultimately led to an non-empty product.  The second
+# was fine.
+print("use_simulation=True")
+b1 = spot.tgba_determinize(b, False, True, True, True)
+assert b1.num_states() == 5
+b1 = spot.remove_fin(spot.dtwa_complement(b1))
+assert spot.otf_product(a, b1).is_empty() == True
+
+print("\nuse_simulation=False")
+b2 = spot.tgba_determinize(b, False, True, False, True)
+assert b2.num_states() == 5
+b2 = spot.remove_fin(spot.dtwa_complement(b2))
+assert spot.otf_product(a, b1).is_empty() == True