Consider the following premises. u_3 -> s_4. s_4 -> u_3. u_3 -> s_4. Can we infer ~(s_4 <-> u_3) from them? If possible, name the inference rules being used at each step.
 From u_3 -> s_4, s_4 -> u_3 we can infer u_3 <-> s_4 via biconditional introduction. From u_3 <-> s_4 we can infer s_4 -> u_3 via biconditional elimination. Finally, from s_4 -> u_3, u_3 -> s_4 we can infer s_4 <-> u_3 via biconditional introduction, which contradicts ~(s_4 <-> u_3). Therefore, the answer is no.