Consider the following premises. For all x_6, if x_6 drives a car, then x_6 is taking a plane. There is at least one x_6 for which x_6 will take a plane if and only if x_6 will drive a car. Can we infer the following from them? If we can, name the inference rule being used: There is at least one x_6 for which if x_6 drives a car, then x_6 is taking a plane.
 From the fact that there is at least one x_6 for which x_6 will take a plane if and only if x_6 will drive a car we can infer that there is at least one x_6 for which if x_6 takes a plane, then x_6 is driving a car via existential biconditional elimination. From the fact that there is at least one x_6 for which if x_6 takes a plane, then x_6 is driving a car, and that for all x_6, if x_6 drives a car, then x_6 is taking a plane we can infer that there is at least one x_6 for which x_6 will take a plane if and only if x_6 will drive a car via existential biconditional introduction. Finally, from the fact that there is at least one x_6 for which x_6 will take a plane if and only if x_6 will drive a car we can infer that there is at least one x_6 for which if x_6 drives a car, then x_6 is taking a plane via existential biconditional elimination. Therefore, the answer is yes.