A -> B1 B2 A -> C1 C2 B1 -> B11 B1 -> B12 B2 -> B21 B2 -> B22 C1 -> C11 C1 -> C12 C2 -> C21 C2 -> C22 B11 -> t B12 -> t B21 -> t B22 -> t C11 -> t C12 -> t C21 -> t C22 -> t