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. s_1 -> u. u -> t. t -> s.
s_1 -> t can be inferred via the transitivity rule. u -> s can be inferred via the transitivity rule. t <-> u can be inferred via the biconditional introduction rule. u <-> t can be inferred via the biconditional introduction rule.