Consider the following premises. There is at least one y_1 for which if y_1 were rich, then y_1 is taking a plane. For all y_1, y_1 will not take a plane. For all y_1, if y_1 takes a plane, then y_1 is rich. Can we infer the following from them? If we can, name the inference rule being used: There is at least one y_1 for which y_1 is not rich.
 From the fact that for all y_1, if y_1 takes a plane, then y_1 is rich, and that there is at least one y_1 for which if y_1 were rich, then y_1 is taking a plane we can infer that there is at least one y_1 for which y_1 will take a plane if and only if y_1 is rich via existential biconditional introduction. Finally, from the fact that there is at least one y_1 for which y_1 will take a plane if and only if y_1 is rich, and that for all y_1, y_1 will not take a plane we can infer that there is at least one y_1 for which y_1 is not rich via existential biconditional elimination. Therefore, the answer is yes.