Consider the following premises. p -> u_4. q -> s. q -> p. s -> q. Can we infer ~(s -> u_4) from them? If possible, name the inference rules being used at each step.
 From q -> p, p -> u_4 we can infer q -> u_4 via transitivity. From q -> s, s -> q we can infer q <-> s via biconditional introduction. From q <-> s we can infer s -> q via biconditional elimination. Finally, from s -> q, q -> u_4 we can infer s -> u_4 via transitivity, which contradicts ~(s -> u_4). Therefore, the answer is no.