Consider the following premises. ~r. u_3 -> r. t <-> u_3. u_3 <-> r. Can we infer ~t from them? If possible, name the inference rules being used at each step.
 From u_3 <-> r we can infer r -> u_3 via biconditional elimination. From u_3 -> r, r -> u_3 we can infer u_3 <-> r via biconditional introduction. From u_3 <-> r, ~r we can infer ~u_3 via biconditional elimination. Finally, from t <-> u_3, ~u_3 we can infer ~t via biconditional elimination. Therefore, the answer is yes.