Consider the following premises. r_6 -> t. u -> u_2. u_1 -> s. t -> u_1. u_2 -> r_6. Can we infer u -> s from them? If possible, name the inference rules being used at each step.
 From t -> u_1, u_1 -> s we can infer t -> s via transitivity. From u -> u_2, u_2 -> r_6 we can infer u -> r_6 via transitivity. From u -> r_6, r_6 -> t we can infer u -> t via transitivity. Finally, from u -> t, t -> s we can infer u -> s via transitivity. Therefore, the answer is yes.