Consider the following premises. For all x_3, if x_3 were an astronaut, then x_3 is not drinking water. There is at least one x_3 for which x_3 is an astronaut if and only if x_3 is a police officer. For all x_3, x_3 will drink water. Can we infer the following from them? If we can, name the inference rule being used: There is at least one x_3 for which x_3 is not a police officer.
 From the fact that for all x_3, if x_3 were an astronaut, then x_3 is not drinking water, and that for all x_3, x_3 will drink water we can infer that for all x_3, x_3 is not an astronaut via universal modus tollens. Finally, from the fact that there is at least one x_3 for which x_3 is an astronaut if and only if x_3 is a police officer, and that for all x_3, x_3 is not an astronaut we can infer that there is at least one x_3 for which x_3 is not a police officer via existential biconditional elimination. Therefore, the answer is yes.