Consider the following premises. u or ~s_4. u or ~w. w_4 or w. ~w_4 or s_4. Can we infer u from them? If possible, name the inference rules being used at each step.
 From w_4 or w, ~w_4 or s_4 we can infer w or s_4 via resolution. From u or ~w we can infer w -> u via rewriting. From u or ~s_4 we can infer s_4 -> u via rewriting. Finally, from w -> u, s_4 -> u, w or s_4 we can infer u via disjunction elimination. Therefore, the answer is yes.