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) <-> S_3(x_18). forall x_18: S(x_18) -> S_4(x_18). forall x_18: U(x_18). exists x_18: S(x_18).
exists x_18: S_4(x_18) can be inferred via the existential modus ponens rule. exists x_18: S(x_18) and U(x_18) can be inferred via the existential conjunction rule. exists x_18: U(x_18) and S(x_18) can be inferred via the existential conjunction rule. forall x_18: S_4(x_18) -> S_3(x_18) can be inferred via the universal biconditional elimination rule. forall x_18: S_3(x_18) -> S_4(x_18) can be inferred via the universal biconditional elimination rule.