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 -> p. ~q_1. p -> q_1. s_4 -> u.
~p can be inferred via the modus tollens rule. u -> q_1 can be inferred via the transitivity rule. s_4 -> p can be inferred via the transitivity rule.