Consider the following premises. ~u_1. w_1 <-> w_4. w_4 -> u_1. Can we infer ~w_1 from them? If possible, name the inference rules being used at each step.
 From w_1 <-> w_4 we can infer w_1 -> w_4 via biconditional elimination. From w_4 -> u_1, ~u_1 we can infer ~w_4 via modus tollens. Finally, from w_1 -> w_4, ~w_4 we can infer ~w_1 via modus tollens. Therefore, the answer is yes.