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: ~p_21. u_4 or q_21. p_21 <-> q_21.
~q_21 -> u_4 can be inferred via the rewriting rule. p_21 -> q_21 can be inferred via the biconditional elimination rule. q_21 -> p_21 can be inferred via the biconditional elimination rule. ~q_21 can be inferred via the biconditional elimination rule.