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. ~R_21(a_21). w_1 -> r. forall x_21: R_21(x_21) or S_21(x_21).
~w_1 can be inferred via the modus tollens rule. ~r and ~R_21(a_21) can be inferred via the conjunction rule. ~R_21(a_21) and ~r can be inferred via the conjunction rule.