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_21(a_21). w -> s_3. forall x_21: R_21(x_21) or S_21(x_21). ~s_3.
~w can be inferred via the modus tollens rule. ~R_21(a_21) and ~s_3 can be inferred via the conjunction rule. ~s_3 and ~R_21(a_21) can be inferred via the conjunction rule.