+−Q 0 1 2 3 +−A a b +−s 0 +−F 1 3 +−t a 0 1 +−t b 0 2 +−t a 1 3 +−t b 1 3 +−t a 2 0 +−t b 2 1 +−t a 3 2 +−t b 3 3