Consider the following premises. s <-> w. ~w. p -> u_2. ~p_20 or p. t_4 -> u_2. p_20 or t_4. Can we infer ~s and u_2 from them? If possible, name the inference rules being used at each step.
 From p_20 or t_4, ~p_20 or p we can infer t_4 or p via resolution. From t_4 -> u_2, p -> u_2, t_4 or p we can infer u_2 via disjunction elimination. From s <-> w, ~w we can infer ~s via biconditional elimination. Finally, from ~s, u_2 we can infer ~s and u_2 via conjunction. Therefore, the answer is yes.