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_21: Q(x_21) <-> S_21(x_21). p <-> q_18. ~S_21(a_21). p -> Q(a_21).
p -> q_18 can be inferred via the biconditional elimination rule. q_18 -> p can be inferred via the biconditional elimination rule. forall x_21: Q(x_21) -> S_21(x_21) can be inferred via the universal biconditional elimination rule. forall x_21: S_21(x_21) -> Q(x_21) can be inferred via the universal biconditional elimination rule.