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 -> u. ~q_4. q_4 -> q. q -> q_4. u -> q.
~q can be inferred via the modus tollens rule. q_4 -> u can be inferred via the transitivity rule. u -> q_4 can be inferred via the transitivity rule. q <-> u can be inferred via the biconditional introduction rule. q_4 <-> q can be inferred via the biconditional introduction rule. q <-> q_4 can be inferred via the biconditional introduction rule. u <-> q can be inferred via the biconditional introduction rule.