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: w_4 -> u. u -> s. q_13 or w_4. u <-> q_13.
w_4 -> s can be inferred via the transitivity rule. u -> q_13 can be inferred via the biconditional elimination rule. q_13 -> u can be inferred via the biconditional elimination rule.