Consider the following premises. There is at least one z for which z is curious if and only if z will drink water. For all z, z will drink water if and only if z is curious. Can we infer the following from them? If we can, name the inference rule being used: There is at least one z for which if z were curious, then z is drinking water.
 From the fact that there is at least one z for which z is curious if and only if z will drink water we can infer that there is at least one z for which if z drinks water, then z is curious via existential biconditional elimination. From the fact that for all z, z will drink water if and only if z is curious we can infer that for all z, if z were curious, then z is drinking water via universal biconditional elimination. From the fact that there is at least one z for which if z drinks water, then z is curious, and that for all z, if z were curious, then z is drinking water we can infer that there is at least one z for which z will drink water if and only if z is curious via existential biconditional introduction. Finally, from the fact that there is at least one z for which z will drink water if and only if z is curious we can infer that there is at least one z for which if z were curious, then z is drinking water via existential biconditional elimination. Therefore, the answer is yes.