Consider the following premises. For all z, z will play squash if and only if z is poor. If Charles plays squash, then Charles is poor. Can we infer the following from them? If we can, name the inference rule being used: Charles is playing squash if and only if Charles is poor.
 From the fact that for all z, z will play squash if and only if z is poor we can infer that for all z, if z were poor, then z is playing squash via universal biconditional elimination. From the fact that for all z, if z were poor, then z is playing squash we can infer that if Charles were poor, then Charles is playing squash via universal instantiation. Finally, from the fact that if Charles plays squash, then Charles is poor, and that if Charles were poor, then Charles is playing squash we can infer that Charles is playing squash if and only if Charles is poor via biconditional introduction. Therefore, the answer is yes.