Consider the following premises. u <-> s. ~t_1. u <-> u_3. r <-> t_1. r <-> u_3. Can we infer s from them? If possible, name the inference rules being used at each step.
 From r <-> t_1, ~t_1 we can infer ~r via biconditional elimination. From r <-> u_3, ~r we can infer ~u_3 via biconditional elimination. From u <-> u_3, ~u_3 we can infer ~u via biconditional elimination. Finally, from u <-> s, ~u we can infer ~s via biconditional elimination, which contradicts s. Therefore, the answer is no.