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