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: r_4 <-> s. w_4 -> u_3. w_4. ~s.
u_3 can be inferred via the modus ponens rule. w_4 and ~s can be inferred via the conjunction rule. ~s and w_4 can be inferred via the conjunction rule. r_4 -> s can be inferred via the biconditional elimination rule. s -> r_4 can be inferred via the biconditional elimination rule. ~r_4 can be inferred via the biconditional elimination rule.