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: ~S_21(a_21). p_18 <-> p_4. p_18 -> Q_1(a_21). forall x_21: Q_1(x_21) <-> S_21(x_21).
p_18 -> p_4 can be inferred via the biconditional elimination rule. p_4 -> p_18 can be inferred via the biconditional elimination rule. forall x_21: Q_1(x_21) -> S_21(x_21) can be inferred via the universal biconditional elimination rule. forall x_21: S_21(x_21) -> Q_1(x_21) can be inferred via the universal biconditional elimination rule.