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: p -> u. p -> ~q_2. q_2. u -> p.
~p can be inferred via the modus tollens rule. u -> ~q_2 can be inferred via the transitivity rule. p <-> u can be inferred via the biconditional introduction rule. u <-> p can be inferred via the biconditional introduction rule.