Consider the following premises. There is at least one z_4 for which z_4 is not a scientist. For all z_4, if z_4 were a taxi driver, then z_4 is a server. For all z_4, if z_4 were a poet, then z_4 is a taxi driver. For all z_4, z_4 is a scientist if and only if z_4 is a server. Can we infer the following from them? If we can, name the inference rule being used: There is at least one z_4 for which z_4 is not a poet.
 From the fact that for all z_4, if z_4 were a poet, then z_4 is a taxi driver, and that for all z_4, if z_4 were a taxi driver, then z_4 is a server we can infer that for all z_4, if z_4 were a poet, then z_4 is a server via universal transitivity. From the fact that for all z_4, z_4 is a scientist if and only if z_4 is a server, and that there is at least one z_4 for which z_4 is not a scientist we can infer that there is at least one z_4 for which z_4 is not a server via existential biconditional elimination. Finally, from the fact that for all z_4, if z_4 were a poet, then z_4 is a server, and that there is at least one z_4 for which z_4 is not a server we can infer that there is at least one z_4 for which z_4 is not a poet via existential modus tollens. Therefore, the answer is yes.