Consider the following premises. For all x_5, x_5 is not a taxi driver. For all x_5, x_5 is happy if and only if x_5 will drive a car. For all x_5, if x_5 drives a car, then x_5 is a taxi driver. Can we infer the following from them? If we can, name the inference rule being used: There is at least one x_5 for which x_5 is happy.
 From the fact that for all x_5, if x_5 drives a car, then x_5 is a taxi driver, and that for all x_5, x_5 is not a taxi driver we can infer that for all x_5, x_5 will not drive a car via universal modus tollens. From the fact that for all x_5, x_5 is happy if and only if x_5 will drive a car we can infer that for all x_5, if x_5 were happy, then x_5 is driving a car via universal biconditional elimination. Finally, from the fact that for all x_5, if x_5 were happy, then x_5 is driving a car, and that for all x_5, x_5 will not drive a car we can infer that for all x_5, x_5 is not happy via universal modus tollens, which contradicts that There is at least one x_5 for which x_5 is happy. Therefore, the answer is no.