Consider the following premises. For all y, if y were poor, then y is a police officer. For all y, y is poor if and only if y 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 y for which the fact that y were poor does not imply that y is a police officer.
 From the fact that for all y, y is poor if and only if y is a police officer we can infer that for all y, if y were a police officer, then y is poor via universal biconditional elimination. From the fact that for all y, if y were a police officer, then y is poor, and that for all y, if y were poor, then y is a police officer we can infer that for all y, y is a police officer if and only if y is poor via universal biconditional introduction. Finally, from the fact that for all y, y is a police officer if and only if y is poor we can infer that for all y, if y were poor, then y is a police officer via universal biconditional elimination, which contradicts that There is at least one y for which the fact that y were poor does not imply that y is a police officer. Therefore, the answer is no.