Consider the following premises. For all z, z will drink water if and only if z is a police officer. For all z, z will drink water if and only if z is a police officer. Can we infer the following from them? If we can, name the inference rule being used: There is at least one z for which if is not true that z is a police officer if and only if z will drink water.
 From the fact that for all z, z will drink water if and only if z is a police officer we can infer that for all z, if z drinks water, then z is a police officer via universal biconditional elimination. From the fact that for all z, z will drink water if and only if z is a police officer we can infer that for all z, if z were a police officer, then z is drinking water via universal biconditional elimination. Finally, from the fact that for all z, if z were a police officer, then z is drinking water, and that for all z, if z drinks water, then z is a police officer we can infer that for all z, z is a police officer if and only if z will drink water via universal biconditional introduction, which contradicts that There is at least one z for which if is not true that z is a police officer if and only if z will drink water. Therefore, the answer is no.