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_13 or u. u -> s. s -> p_1. s <-> q_13.
u -> p_1 can be inferred via the transitivity rule. s -> q_13 can be inferred via the biconditional elimination rule. q_13 -> s can be inferred via the biconditional elimination rule.