Consider the following premises. p_5 <-> s_4. s_4 -> p_5. ~p_5. Can we infer ~s_4 from them? If possible, name the inference rules being used at each step.
 From p_5 <-> s_4 we can infer p_5 -> s_4 via biconditional elimination. From s_4 -> p_5, p_5 -> s_4 we can infer s_4 <-> p_5 via biconditional introduction. Finally, from s_4 <-> p_5, ~p_5 we can infer ~s_4 via biconditional elimination. Therefore, the answer is yes.