simplify: GF(f)=GF(dnf(f)) FG(f)=FG(cnf(f))

These rules come from Delag's paper, and help some cases
in issue #385.

* spot/tl/simplify.cc: Implement the simplification.
* doc/tl/tl.tex, NEWS: Document it.
* tests/core/385.test: New file.
* tests/Makefile.am: Add it.
* tests/core/reduccmp.test: More tests.
* tests/core/ltl2tgba2.test: Adjust one improved case.
* tests/python/automata.ipynb, tests/python/twagraph-internals.ipynb:
Adjust expected output, as the cnf/dnf reorder some subformulas.

tests/core/385.test

@@ -0,0 +1,45 @@
+#!/bin/sh
+# -*- coding: utf-8 -*-
+# Copyright (C) 2019 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/>.
+
+. ./defs
+
+set -e
+
+# Formulas from issue #385.
+
+ltl2tgba -G --stats=%s,%f> out <<EOF
+GFX((e & XXXa) -> (e & X!d))
+G(Gc | Fa | F!G(c | Ge))
+Ge | XGF(Ge & X(c & Fd))
+G!GXXe -> GF(b & c & Gc)
+G!(Gd & (c | Fb))
+XF(F(Gc & Xb) -> a)
+EOF
+
+# Some of the following are still not optimal.
+cat >expected <<EOF
+1,GF(!a | !d | !e)
+6,G(Gc | F(a | (!c & F!e)))
+2,Ge | G(Fd & FGe & Fc)
+1,F(G(Fb & FGc) | Ge)
+4,G(F!d | (!c & G!b))
+3,F(GF!c | G!b) | XFa
+EOF
+diff out expected