Consider the following premises. s_1 -> t. q_6 -> s_1. u -> q_6. ~t. Can we infer u from them?
 From u -> q_6, q_6 -> s_1 we can infer u -> s_1. From s_1 -> t, ~t we can infer ~s_1. Finally, from u -> s_1, ~s_1 we can infer ~u, which contradicts u. Therefore, the answer is no.