Consider the following premises. For all z, z will play squash or z will not drink water. There is at least one z for which z will write a letter if and only if z will drink water. Can we infer the following from them? If we can, name the inference rule being used: For all z, the fact that z writes a letter does not imply that z is playing squash.
 From the fact that there is at least one z for which z will write a letter if and only if z will drink water we can infer that there is at least one z for which if z writes a letter, then z is drinking water via existential biconditional elimination. Finally, from the fact that for all z, z will play squash or z will not drink water we can infer that for all z, if z drinks water, then z is playing squash via universal rewriting, which contradicts that For all z, the fact that z writes a letter does not imply that z is playing squash. Therefore, the answer is no.