Consider the following premises. u_3. u -> u_3. u or ~u_3. Can we infer u from them?
 From u or ~u_3 we can infer u_3 -> u. From u_3 -> u, u -> u_3 we can infer u_3 <-> u. From u_3 <-> u we can infer u_3 -> u. Finally, from u_3 -> u, u_3 we can infer u. Therefore, the answer is yes.