acc: make sure unit_propagate preserve the number of sets

* spot/twa/acc.hh: Here.
* tests/core/accsimpl.test, tests/core/ltl2tgba2.test: Add test cases.

tests/core/accsimpl.test

@@ -1,6 +1,6 @@
 #!/bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2018 Laboratoire de Recherche et Développement de
+# Copyright (C) 2018, 2020 Laboratoire de Recherche et Développement de
 # l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
@@ -51,3 +51,25 @@ EOF
 autfilt --simplify-acceptance 325 > 325s
 exp='3 Fin(0) | (Fin(1) & Inf(2))'
 test "$exp" = "`autfilt --stats='%a %g' 325s`"
+
+# This automaton used to cause some issue at some point, because
+# simplify_acceptance would simplify the acceptance condition without
+# removing the unnecessary set from the resulting automaton.
+cat >frenkin <<EOF
+HOA: v1
+States: 1
+Start: 0
+AP: 2 "a" "b"
+Acceptance: 4 Fin(0) & (Fin(1) | (Inf(2)&Inf(3)))
+properties: trans-labels explicit-labels trans-acc complete
+properties: deterministic stutter-invariant
+--BODY--
+State: 0
+[!0&1] 0
+[!0&!1] 0 {1 3}
+[0&1] 0 {0 1 2}
+[0&!1] 0 {0 1 2 3}
+--END--
+EOF
+autcross --language-preserved 'autfilt --simplify-acc' -F frenkin
+test 1,4,2 = "`autfilt --simplify-acc frenkin --stats=%s,%e,%a`"