Consider the following premises. ~p_1. s_4 -> p_1. p_1 -> s_4. Can we infer ~s_4 from them? If possible, name the inference rules being used at each step.
 From p_1 -> s_4, s_4 -> p_1 we can infer p_1 <-> s_4 via biconditional introduction. Finally, from p_1 <-> s_4, ~p_1 we can infer ~s_4 via biconditional elimination. Therefore, the answer is yes.