sbacc: fix a serious bug

Reported by Thibaud Michaud

* spot/twaalgos/sbacc.cc: Do not label rejecting SCCs with the empty
mark, as it might be accepting.
* tests/core/sbacc.test: Add test cases.
* NEWS: Mention the bug.

tests/core/sbacc.test

@@ -159,3 +159,68 @@ randltl --weak-fairness -n 20 2 |
              "$ltl2tgba -H %f | $autfilt -H >%O"
 
 test 4 = `ltl2tgba -S 'F(a & X(!a &Xb))' --any --stats=%s`
+
+
+# Test case from Thibaud Michaud.  This used to fail, because sbacc()
+# would remove acceptance marks from all rejecting SCCS...
+cat >tm.hoa <<EOF
+HOA: v1
+States: 5
+Start: 1
+AP: 4 "req" "go" "ack" "cancel"
+acc-name: parity max odd 3
+Acceptance: 3 Fin(2) & (Inf(1) | Fin(0))
+properties: trans-labels explicit-labels trans-acc colored complete
+properties: deterministic
+--BODY--
+State: 0
+[t] 1 {0}
+State: 1
+[!0] 0 {0}
+[0] 2 {0}
+State: 2
+[2] 1 {0}
+[!2] 3 {0}
+State: 3
+[t] 4 {0}
+State: 4
+[2] 1 {2}
+[!2] 3 {1}
+--END--
+EOF
+autfilt --sbacc tm.hoa > out.hoa
+autfilt -q --equivalent-to out.hoa tm.hoa
+
+# Tautological acceptances are converted to "t"
+cat >taut.hoa <<EOF
+HOA: v1
+States: 2
+Start: 1
+AP: 0
+Acceptance: 2 Fin(0) | Inf(0) | Fin(1)
+--BODY--
+State: 0
+[t] 1 {0}
+State: 1
+[t] 0
+[t] 1 {1}
+--END--
+EOF
+cat >expect.hoa <<EOF
+HOA: v1
+States: 2
+Start: 1
+AP: 0
+acc-name: all
+Acceptance: 0 t
+properties: trans-labels explicit-labels state-acc complete weak
+--BODY--
+State: 0
+[t] 1
+State: 1
+[t] 0
+[t] 1
+--END--
+EOF
+autfilt --sbacc taut.hoa > out.hoa
+diff out.hoa expect.hoa