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: u_3 -> w. w -> s_1. s_1 -> w. s_1 -> w_2. w_2 -> u_3.
u_3 -> s_1 can be inferred via the transitivity rule. w -> w_2 can be inferred via the transitivity rule. s_1 -> u_3 can be inferred via the transitivity rule. w_2 -> w can be inferred via the transitivity rule. w <-> s_1 can be inferred via the biconditional introduction rule. s_1 <-> w can be inferred via the biconditional introduction rule.