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: u -> p_13. u or q. p_13 <-> q. s_1 -> ~p_13.
p_13 -> q can be inferred via the biconditional elimination rule. q -> p_13 can be inferred via the biconditional elimination rule.