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 <-> u_2. ~u_2. r_4 -> w_3. w_3 -> r_4.
r_4 <-> w_3 can be inferred via the biconditional introduction rule. w_3 <-> r_4 can be inferred via the biconditional introduction rule. r_4 -> u_2 can be inferred via the biconditional elimination rule. u_2 -> r_4 can be inferred via the biconditional elimination rule. ~r_4 can be inferred via the biconditional elimination rule.