is_unambiguous: rewrite more efficiently

Avoid calling scc_info::determine_unknown_acceptance on the product, as
suggested in #188.

* spot/twaalgos/isunamb.cc (is_unambiguous): Rewrite.
* tests/core/unambig.test: Add the automaton from #188.
* NEWS: Mention the improved function.
* spot/twaalgos/mask.cc,
spot/twaalgos/mask.hh (mask_keep_accessible_states): New function.

tests/core/unambig.test

@@ -244,3 +244,47 @@ cat >expected <<EOF
 1
 EOF
 diff expected ltlcross.res
+
+
+# This automaton was supplied by František Blahoudek, and is discussed
+# in #188.  Our initial implementation of is_unambiguous() on this
+# automaton used to take 38s, while the new version takes 0.2s.  Using
+# valgrind on this is a way to ensure that the test is procedure
+# enough: it would have been too long on the previous version.
+
+cat >smaller.hoa<<EOF
+HOA: v1
+States: 7
+Start: 1
+AP: 2 "a" "b"
+Acceptance: 12 (Inf(11) | Fin(5)) & (Inf(10) | Fin(4)) & (Inf(9) |
+  Fin(3)) & (Inf(8) | Fin(2)) & (Inf(7) | Fin(1)) & (Inf(6) | Fin(0))
+properties: trans-labels explicit-labels trans-acc complete
+--BODY--
+State: 0
+[t] 0
+State: 1
+[!0] 1 {0}
+[0] 2
+[0&1] 3
+State: 2
+[t] 2 {1}
+[1] 3
+State: 3
+[t] 3 {2}
+[!1] 4
+State: 4
+[t] 4 {3}
+[1] 5
+State: 5
+[t] 5 {4}
+[!1] 6
+State: 6
+[1] 0 {11}
+[!1] 6 {5}
+--END--
+EOF
+
+run 1 autfilt -q --is-unambiguous smaller.hoa
+
+true