Consider the following premises. u -> q_1. t -> t_1. ~t_1. q_1 <-> t. Can we infer u from them? If possible, name the inference rules being used at each step.
 From t -> t_1, ~t_1 we can infer ~t via modus tollens. From q_1 <-> t, ~t we can infer ~q_1 via biconditional elimination. Finally, from u -> q_1, ~q_1 we can infer ~u via modus tollens, which contradicts u. Therefore, the answer is no.