introduce delay_branching_here

This is motivated by an example sent by Edmond Irani Liu,
that will be tested in next patch.

* spot/twaalgos/dbranch.cc, spot/twaalgos/dbranch.hh: New files.
* python/spot/impl.i, spot/twaalgos/Makefile.am: Add them.
* spot/twaalgos/translate.cc: Call delay_branching_here
unconditionally.
* spot/twa/twagraph.cc (defrag_states): Do not assume
that games are alternating.
* tests/core/genltl.test: Adjust expected numbers.
* tests/python/dbranch.py: New file.
* tests/Makefile.am: Add it.

spot/twaalgos/dbranch.hh

@@ -0,0 +1,36 @@
+// -*- coding: utf-8 -*-
+// Copyright (C) 2022 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/>.
+
+#pragma once
+
+#include <spot/twa/twagraph.hh>
+
+namespace spot
+{
+  /// \ingroup twa_algorithms
+  /// \brief Merge states to delay
+  ///
+  /// If a state (x) has two outgoing transitions (x,l,m,y) and
+  /// (x,l,m,z) going to states (x) and (y) that have no other
+  /// incoming edges, then (y) and (z) can be merged (keeping the
+  /// union of their outgoing destinations).
+  ///
+  /// \return true iff the automaton was modified.
+  SPOT_API bool delay_branching_here(const twa_graph_ptr& aut);
+}