Consider the following premises. For all z, z is a janitor if and only if z is excited. For all z, if z takes a plane, then z is not a janitor. For all z, z is excited. Can we infer the following from them? There is at least one z for which z will take a plane.
 From the fact that for all z, z is a janitor if and only if z is excited we can infer that for all z, if z were excited, then z is a janitor. From the fact that for all z, if z were excited, then z is a janitor, and that for all z, z is excited we can infer that for all z, z is a janitor. Finally, from the fact that for all z, if z takes a plane, then z is not a janitor, and that for all z, z is a janitor we can infer that for all z, z will not take a plane, which contradicts that There is at least one z for which z will take a plane. Therefore, the answer is no.