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: exists z_2: S_2(z_2) -> U_4(z_2). forall z_2: U_4(z_2) -> S_2(z_2). forall z_2: S_2(z_2) -> U_4(z_2).
forall z_2: U_4(z_2) <-> S_2(z_2) can be inferred via the universal biconditional introduction rule. forall z_2: S_2(z_2) <-> U_4(z_2) can be inferred via the universal biconditional introduction rule. exists z_2: S_2(z_2) <-> U_4(z_2) can be inferred via the existential biconditional introduction rule. exists z_2: U_4(z_2) <-> S_2(z_2) can be inferred via the existential biconditional introduction rule.