tl: implement to_delta2()

* spot/tl/delta2.cc, spot/tl/delta2.hh: New files.
* spot/tl/Makefile.am: Add them.
* python/spot/impl.i: Include delta2.hh.
* tests/python/delta2.py: New file.
* tests/Makefile.am: Add it.
* NEWS: Mention the change.

spot/tl/delta2.hh

@@ -0,0 +1,42 @@
+// -*- coding: utf-8 -*-
+// Copyright (C) by the Spot authors, see the AUTHORS file for details.
+//
+// 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/tl/formula.hh>
+#include <spot/tl/simplify.hh>
+
+namespace spot
+{
+  /// \ingroup tl_rewriting
+  /// \brief Convert an LTL formula to Δ₂
+  ///
+  /// This implement LTL rewriting rules as given by
+  /// \cite esparza.24.acm
+  ///
+  /// Only LTL operators are supported, PSL operators
+  /// will be left untouched.
+  ///
+  /// If \a tls is given, it will be used to simplify formulas and
+  /// puts formulas in negative normal form.  If \a tls is not
+  /// given, a temporary simplifier will be created.
+  ///
+  /// No transformation is attempted if the input is already Δ₂.
+  SPOT_API formula
+  to_delta2(formula f, tl_simplifier* tls = nullptr);
+}