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. u_2 -> ~r. ~u. s <-> u.
~u_2 can be inferred via the modus tollens rule. r and ~u can be inferred via the conjunction rule. ~u and r can be inferred via the conjunction rule. s -> u can be inferred via the biconditional elimination rule. u -> s can be inferred via the biconditional elimination rule. ~s can be inferred via the biconditional elimination rule.