tl: implement suffix operator normal form

* spot/tl/Makefile.am: New sonf files
* spot/tl/sonf.cc, spot/tl/sonf.hh: Here.
* python/spot/impl.i: include sonf.hh header
* doc/spot.bib: add entry for the SONF paper
* tests/Makefile.am: new python tests
* tests/python/formulas.ipynb: show sample usage
* tests/python/sonf.py: test automata equivalence before/after SONF
* NEWS: mention the change

tests/Makefile.am

@@ -445,6 +445,7 @@ TESTS_python = \
   python/setxor.py \
   python/simplacc.py \
   python/simstate.py \
+  python/sonf.py \
   python/split.py \
   python/streett_totgba.py \
   python/streett_totgba2.py \

tests/python/formulas.ipynb

@@ -976,6 +976,62 @@
     "print(ap)        # print as a string\n",
     "display(ap)      # LaTeX-style, for notebooks"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Converting to Suffix Operator Normal Form:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 27,
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/latex": [
+       "$\\mathsf{G} (\\{x^{\\star}\\}\\mathrel{\\Box\\kern-1.7pt\\raise.4pt\\hbox{$\\mathord{\\rightarrow}$}} \\mathsf{F} a)$"
+      ],
+      "text/plain": [
+       "spot.formula(\"G({x[*]}[]-> Fa)\")"
+      ]
+     },
+     "metadata": {},
+     "output_type": "display_data"
+    },
+    {
+     "data": {
+      "text/latex": [
+       "$\\mathsf{G} \\mathit{sonf\\_}_{0} \\land \\mathsf{G} (\\lnot \\mathit{sonf\\_}_{1} \\lor \\mathsf{F} a) \\land \\mathsf{G} (\\lnot \\mathit{sonf\\_}_{0} \\lor (\\{x^{\\star}\\}\\mathrel{\\Box\\kern-1.7pt\\raise.4pt\\hbox{$\\mathord{\\rightarrow}$}} \\mathit{sonf\\_}_{1}))$"
+      ],
+      "text/plain": [
+       "spot.formula(\"Gsonf_0 & G(!sonf_1 | Fa) & G(!sonf_0 | ({x[*]}[]-> sonf_1))\")"
+      ]
+     },
+     "metadata": {},
+     "output_type": "display_data"
+    },
+    {
+     "data": {
+      "text/plain": [
+       "('sonf_0', 'sonf_1')"
+      ]
+     },
+     "metadata": {},
+     "output_type": "display_data"
+    }
+   ],
+   "source": [
+    "f = spot.formula('G({x*} []-> Fa)')\n",
+    "display(f)\n",
+    "\n",
+    "# In addition to the formula, returns a list of newly introduced APs\n",
+    "f, aps = spot.suffix_operator_normal_form(f, 'sonf_')\n",
+    "display(f)\n",
+    "display(aps)"
+   ]
   }
  ],
  "metadata": {

tests/python/sonf.py

@@ -0,0 +1,41 @@
+# -*- mode: python; coding: utf-8 -*-
+# Copyright (C) 2020 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/>.
+
+import spot
+
+formulas = """\
+{x[*]}[]-> F({y[*]}<>-> GFz)
+<>(({{p12}[*0..3]}[]-> ((p9) || (!(p17)))) V ((true) U (p17)))
+{{true} || {[*0]}}[]-> (false)
+{{p14} & {{p0}[*]}}[]-> (p11)
+{{{!{p6}} -> {!{p3}}}[*]}[]-> ((p3)V((p3) || ((X((false))) && ((p2)V(p18)))))
+"""
+
+for f1 in formulas.splitlines():
+    f1 = spot.formula(f1)
+    a1 = spot.translate(f1)
+
+    f2, aps = spot.suffix_operator_normal_form(f1, 'sonf_')
+    a2 = spot.translate(f2)
+    rm = spot.remove_ap()
+    for ap in aps:
+        rm.add_ap(ap)
+    a2 = rm.strip(a2)
+
+    assert(spot.are_equivalent(a1, a2))