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: r. u_4 -> ~q_13. q_13. w_4 -> u_4. u_4 -> w_4.
~u_4 can be inferred via the modus tollens rule. w_4 -> ~q_13 can be inferred via the transitivity rule. r and q_13 can be inferred via the conjunction rule. q_13 and r can be inferred via the conjunction rule. w_4 <-> u_4 can be inferred via the biconditional introduction rule. u_4 <-> w_4 can be inferred via the biconditional introduction rule.