Consider the following premises. There is at least one z for which z is a writer or z will not work. For all z, z will work or z is not a writer. Can we infer the following from them? If we can, name the inference rule being used: There is at least one z for which z is a writer if and only if z will work.
 From the fact that for all z, z will work or z is not a writer we can infer that for all z, if z were a writer, then z is working via universal rewriting. From the fact that there is at least one z for which z is a writer or z will not work we can infer that there is at least one z for which if z works, then z is a writer via existential rewriting. Finally, from the fact that for all z, if z were a writer, then z is working, and that there is at least one z for which if z works, then z is a writer we can infer that there is at least one z for which z is a writer if and only if z will work via existential biconditional introduction. Therefore, the answer is yes.