Consider the following premises. t_2 <-> q_11. u -> t_1. ~q_11. t_1 -> u. t_1 -> t_2. Can we infer u from them? If possible, name the inference rules being used at each step.
 From t_2 <-> q_11, ~q_11 we can infer ~t_2 via biconditional elimination. From t_1 -> t_2, ~t_2 we can infer ~t_1 via modus tollens. From t_1 -> u, u -> t_1 we can infer t_1 <-> u via biconditional introduction. Finally, from t_1 <-> u, ~t_1 we can infer ~u via biconditional elimination, which contradicts u. Therefore, the answer is no.