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 -> u. u -> w_2. u -> w_2.
w_2 <-> u can be inferred via the biconditional introduction rule. w_2 <-> u can be inferred via the biconditional introduction rule. u <-> w_2 can be inferred via the biconditional introduction rule. u <-> w_2 can be inferred via the biconditional introduction rule.