image/svg+xml(S, 0, 4) (S ::= A B C • D ,0,3) (D, 3, 4) (S ::= A B • C D ,0,2) (C, 2, 3) (A, 0, 1) (B, 1, 2) (a, 0, 1) (b, 1, 2) (c, 2, 3) (d, 3, 4)