Consider the following premises. u <-> r_11. q -> r_11. t_2 -> r_11. q or t_2. Can we infer ~u from them? If possible, name the inference rules being used at each step.
 From q -> r_11, t_2 -> r_11, q or t_2 we can infer r_11 via disjunction elimination. From u <-> r_11 we can infer r_11 -> u via biconditional elimination. Finally, from r_11 -> u, r_11 we can infer u via modus ponens, which contradicts ~u. Therefore, the answer is no.