remprop: reset no-terminal property

Reported by Yong Li.

* spot/twaalgos/remprop.cc: Here.
* tests/python/removeap.py: New test case.
* tests/Makefile.am: Add it.
* NEWS: Document the issue.
* THANKS: Add Yong Li.

NEWS

@@ -14,6 +14,10 @@ New in spot 2.7.4.dev (not yet released)
     C++ code for translating certains formulas to be noticeably slower
     than the equivalent call to the ltl2tgba binary.
 
+  - The remove_ap algorithm was preserving the "terminal property" of
+    automata, but it is possible that a non-terminal input produces a
+    terminal output after some propositions are removed.
+
 New in spot 2.7.4 (2019-04-27)
 
   Bugs fixed:

THANKS

@@ -50,4 +50,5 @@ Victor Khomenko
 Vitus Lam
 Yann Thierry-Mieg
 Yannick Molinghen
+Yong Li
 Yuri Victorovich

spot/twaalgos/remprop.cc

 // -*- coding: utf-8 -*-
-// Copyright (C) 2015-2018 Laboratoire de Recherche et Développement
+// Copyright (C) 2015-2019 Laboratoire de Recherche et Développement
 // de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -162,6 +162,10 @@ namespace spot
           }
       }
 
+    if (res->prop_terminal().is_false())
+      // Non-terminal automata could become terminal.
+      res->prop_terminal(trival::maybe());
+
     transform_accessible(aut, res, [&](unsigned, bdd& cond,
                                        acc_cond::mark_t&, unsigned)
                          {

tests/Makefile.am

@@ -398,6 +398,7 @@ TESTS_python = \
   python/randgen.py \
   python/relabel.py \
   python/remfin.py \
+  python/removeap.py \
   python/satmin.py \
   python/sbacc.py \
   python/sccfilter.py \

tests/python/removeap.py

+# -*- mode: python; coding: utf-8 -*-
+# Copyright (C) 2019  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
+
+
+aut = spot.translate('a U (c & Gb)')
+assert not spot.is_terminal_automaton(aut)
+assert aut.prop_terminal().is_false()
+rem = spot.remove_ap()
+rem.add_ap("b")
+aut = rem.strip(aut)
+assert not aut.prop_terminal().is_false()
+assert spot.is_terminal_automaton(aut)
+assert aut.prop_terminal().is_true()
+aut = rem.strip(aut)
+assert aut.prop_terminal().is_true()