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 x_18: S_4(x_18). forall x_18: W_1(x_18) -> U_3(x_18). forall x_18: S_4(x_18) <-> U_3(x_18).
forall x_18: S_4(x_18) -> U_3(x_18) can be inferred via the universal biconditional elimination rule. forall x_18: U_3(x_18) -> S_4(x_18) can be inferred via the universal biconditional elimination rule.