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: p -> r. r_4 -> w. ~r. u -> p. r_4.
w can be inferred via the modus ponens rule. ~p can be inferred via the modus tollens rule. u -> r can be inferred via the transitivity rule. ~r and r_4 can be inferred via the conjunction rule. r_4 and ~r can be inferred via the conjunction rule.