Consider the following premises. For all z_4, z_4 will drink water if and only if z_4 is a janitor. For all z_4, z_4 will not take a plane. For all z_4, z_4 will drink water if and only if z_4 will take a plane. For all z_4, if z_4 were a janitor, then z_4 is drinking water. Can we infer the following from them? If we can, name the inference rule being used: For all z_4, z_4 is not a janitor.
 From the fact that for all z_4, z_4 will drink water if and only if z_4 will take a plane, and that for all z_4, z_4 will not take a plane we can infer that for all z_4, z_4 will not drink water via universal biconditional elimination. From the fact that for all z_4, z_4 will drink water if and only if z_4 is a janitor we can infer that for all z_4, if z_4 drinks water, then z_4 is a janitor via universal biconditional elimination. From the fact that for all z_4, if z_4 were a janitor, then z_4 is drinking water, and that for all z_4, if z_4 drinks water, then z_4 is a janitor we can infer that for all z_4, z_4 is a janitor if and only if z_4 will drink water via universal biconditional introduction. From the fact that for all z_4, z_4 is a janitor if and only if z_4 will drink water we can infer that for all z_4, if z_4 were a janitor, then z_4 is drinking water via universal biconditional elimination. Finally, from the fact that for all z_4, if z_4 were a janitor, then z_4 is drinking water, and that for all z_4, z_4 will not drink water we can infer that for all z_4, z_4 is not a janitor via universal modus tollens. Therefore, the answer is yes.