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