Consider the following premises. p_11 -> w_2. w_4 -> u. w_2 -> p_11. w_2 -> u. w_4 or p_11. Can we infer u from them? If possible, name the inference rules being used at each step.
 From p_11 -> w_2, w_2 -> p_11 we can infer p_11 <-> w_2 via biconditional introduction. From p_11 <-> w_2 we can infer p_11 -> w_2 via biconditional elimination. From p_11 -> w_2, w_2 -> u we can infer p_11 -> u via transitivity. Finally, from w_4 -> u, p_11 -> u, w_4 or p_11 we can infer u via disjunction elimination. Therefore, the answer is yes.