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: q_15 -> r_18. p_4 or q_18. q_18 -> q_15. ~p_4 or r_18. r_18 -> q_15.
q_18 -> r_18 can be inferred via the transitivity rule. q_18 or r_18 can be inferred via the resolution rule. q_15 <-> r_18 can be inferred via the biconditional introduction rule. r_18 <-> q_15 can be inferred via the biconditional introduction rule.