What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? exists z_2: U(z_2) -> S_1(z_2). forall z_2: S_1(z_2) -> U(z_2).
exists z_2: U(z_2) <-> S_1(z_2). exists z_2: S_1(z_2) <-> U(z_2).