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 -> w_3. u <-> r_3. w_3 -> u.
u <-> w_3 can be inferred via the biconditional introduction rule. w_3 <-> u can be inferred via the biconditional introduction rule. u -> r_3 can be inferred via the biconditional elimination rule. r_3 -> u can be inferred via the biconditional elimination rule.