Consider the following premises. There is at least one y for which if y were a writer, then y is taking a plane. For all y, y will take a plane if and only if y is a writer. Can we infer the following from them? If we can, name the inference rule being used: There is at least one y for which y is a writer if and only if y will take a plane.
 From the fact that for all y, y will take a plane if and only if y is a writer we can infer that for all y, if y takes a plane, then y is a writer via universal biconditional elimination. Finally, from the fact that there is at least one y for which if y were a writer, then y is taking a plane, and that for all y, if y takes a plane, then y is a writer we can infer that there is at least one y for which y is a writer if and only if y will take a plane via existential biconditional introduction. Therefore, the answer is yes.