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_2 or p. p_2 -> q_4. q_4 -> p_18. p_18 <-> u. ~u.
p_18 -> u can be inferred via the biconditional elimination rule. u -> p_18 can be inferred via the biconditional elimination rule. ~p_18 can be inferred via the biconditional elimination rule.