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. p -> ~t. u <-> q_9. u -> p.
~p can be inferred via the modus tollens rule. u -> q_9 can be inferred via the biconditional elimination rule. q_9 -> u can be inferred via the biconditional elimination rule.