Consider the following premises. q_1 -> p_6. r -> q_1. p_6 -> u. u -> r_6. Can we infer ~(r -> r_6) from them?
 From p_6 -> u, u -> r_6 we can infer p_6 -> r_6. From r -> q_1, q_1 -> p_6 we can infer r -> p_6. Finally, from r -> p_6, p_6 -> r_6 we can infer r -> r_6, which contradicts ~(r -> r_6). Therefore, the answer is no.