Consider the following premises. s -> u. ~s. q <-> u. u -> r_4. r_4 -> s. Can we infer q from them? If possible, name the inference rules being used at each step.
 From u -> r_4, r_4 -> s we can infer u -> s via transitivity. From s -> u, u -> s we can infer s <-> u via biconditional introduction. From s <-> u, ~s we can infer ~u via biconditional elimination. Finally, from q <-> u, ~u we can infer ~q via biconditional elimination, which contradicts q. Therefore, the answer is no.