Consider the following premises. q -> u_1. s_2 -> q. ~u_1. r <-> s_2. Can we infer r from them? If possible, name the inference rules being used at each step.
 From q -> u_1, ~u_1 we can infer ~q via modus tollens. From s_2 -> q, ~q we can infer ~s_2 via modus tollens. Finally, from r <-> s_2, ~s_2 we can infer ~r via biconditional elimination, which contradicts r. Therefore, the answer is no.