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: R_21(x_21) <-> S_21(x_21). S_21(e) <-> p_2. ~R_21(e).
S_21(e) -> p_2 can be inferred via the biconditional elimination rule. p_2 -> S_21(e) can be inferred via the biconditional elimination rule. forall x_21: R_21(x_21) -> S_21(x_21) can be inferred via the universal biconditional elimination rule. forall x_21: S_21(x_21) -> R_21(x_21) can be inferred via the universal biconditional elimination rule.