python: add bindings for bdd_to_formula()

Follow-up to an email from Ayrat Khalimov.

* python/spot/impl.i: Include twa/formula2bdd.hh.
* python/spot/__init__.py: Make the dictionnary
optional.
* spot/twa/formula2bdd.cc: Throw an exception instead of asserting.
* tests/python/bdditer.py: New file.
* tests/Makefile.am: Add it.
* NEWS: Update.

tests/python/bdditer.py

@@ -0,0 +1,62 @@
+# -*- mode: python; coding: utf-8 -*-
+# Copyright (C) 2017 Laboratoire de Recherche et Développement de 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/>.
+
+
+# Tests different ways to explore a BDD label, as discussed in
+# http://lists.lrde.epita.fr/pipermail/spot/2017q1/000117.html
+
+import spot
+import buddy
+import sys
+
+run = spot.translate('a & !b').accepting_run()
+b = run.prefix[0].label
+c = buddy.bdd_satone(b)
+assert c != buddy.bddfalse
+res = []
+while c != buddy.bddtrue:
+    var = buddy.bdd_var(c)
+    h = buddy.bdd_high(c)
+    if h == buddy.bddfalse:
+        res.append(-var)
+        c = buddy.bdd_low(c)
+    else:
+        res.append(var)
+        c = h
+
+assert res == [0, -1]
+
+res2 = []
+for i in run.aut.ap():
+    res2.append((i, run.aut.register_ap(i)))
+assert str(res2) == '[(a, 0), (b, 1)]'
+
+
+f = spot.bdd_to_formula(b)
+assert f._is(spot.op_And)
+assert f[0]._is(spot.op_ap)
+assert f[1]._is(spot.op_Not)
+assert f[1][0]._is(spot.op_ap)
+assert str(f) == 'a & !b'
+
+try:
+    f = spot.bdd_to_formula(b, spot.make_bdd_dict())
+    sys.exit(2)
+except RuntimeError as e:
+    assert "not in the dictionary" in str(e)