twa_graph: add basic support for alternation

This only allows creating universal edges, and reading the associated
destinations.

* spot/twa/twagraph.hh (new_univ_edges, univ_dests, is_alternating): New
function.
* python/spot/impl.i: Add Python bindings.
* tests/python/alternating.py: New file.
* tests/Makefile.am: Add it.

tests/python/alternating.py

@@ -0,0 +1,52 @@
+#!/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/>.
+
+import spot
+import buddy
+
+aut = spot.make_twa_graph(spot._bdd_dict)
+
+p1 = buddy.bdd_ithvar(aut.register_ap("p1"))
+p2 = buddy.bdd_ithvar(aut.register_ap("p2"))
+
+m = aut.set_buchi()
+
+aut.new_states(3)
+
+aut.set_init_state(0)
+aut.new_univ_edge(0, [1, 2], p1, m)
+aut.new_univ_edge(0, [0, 1], p2)
+aut.new_univ_edge(1, [0, 2, 1], p1 & p2)
+aut.new_edge(2, 2, p1 | p2)
+
+tr = [(s, [[x for x in aut.univ_dests(i)] for i in aut.out(s)])
+      for s in range(3)]
+print(tr)
+assert [(0, [[1, 2], [0, 1]]), (1, [[0, 2, 1]]), (2, [[2]])] == tr
+assert aut.is_alternating()
+
+received = False
+try:
+    for i in aut.out(0):
+        for j in aut.out(i.dst):
+            pass
+except RuntimeError:
+    received = True
+assert received