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: w_2 -> t. t -> s_2. u -> s_2. s_2 -> u.
w_2 -> s_2 can be inferred via the transitivity rule. t -> u can be inferred via the transitivity rule. u <-> s_2 can be inferred via the biconditional introduction rule. s_2 <-> u can be inferred via the biconditional introduction rule.