What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? Name the inference rule being used: forall z_4: ~S(z_4). forall z_4: U(z_4) -> S(z_4). forall z_4: S(z_4) -> U(z_4).
forall z_4: ~U(z_4) can be inferred via the universal modus tollens rule. forall z_4: U(z_4) <-> S(z_4) can be inferred via the universal biconditional introduction rule. forall z_4: S(z_4) <-> U(z_4) can be inferred via the universal biconditional introduction rule.