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: t <-> q_3. ~p_4. u <-> q_3. u -> p_4.
~u can be inferred via the modus tollens rule. t -> q_3 can be inferred via the biconditional elimination rule. u -> q_3 can be inferred via the biconditional elimination rule. q_3 -> t can be inferred via the biconditional elimination rule. q_3 -> u can be inferred via the biconditional elimination rule.