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 -> u_1. s or u_1. ~s or r_9. u_1 -> t. ~t.
~u_1 can be inferred via the modus tollens rule. u_1 or r_9 can be inferred via the resolution rule. t <-> u_1 can be inferred via the biconditional introduction rule. u_1 <-> t can be inferred via the biconditional introduction rule.