Consider the following premises. ~s_1. s_1 <-> u. t_3 -> u. t_3 <-> s_1. Can we infer u from them? If possible, name the inference rules being used at each step.
 From t_3 <-> s_1 we can infer s_1 -> t_3 via biconditional elimination. From s_1 -> t_3, t_3 -> u we can infer s_1 -> u via transitivity. From s_1 <-> u we can infer u -> s_1 via biconditional elimination. From s_1 -> u, u -> s_1 we can infer s_1 <-> u via biconditional introduction. Finally, from s_1 <-> u, ~s_1 we can infer ~u via biconditional elimination, which contradicts u. Therefore, the answer is no.