Consider the following premises. For all x_11, x_11 will play tennis if and only if x_11 will take a plane. For all x_11, x_11 will take a plane if and only if x_11 will play tennis. Can we infer the following from them? If David plays tennis, then David is taking a plane.
 From the fact that for all x_11, x_11 will take a plane if and only if x_11 will play tennis we can infer that for all x_11, if x_11 plays tennis, then x_11 is taking a plane. From the fact that for all x_11, x_11 will play tennis if and only if x_11 will take a plane we can infer that for all x_11, if x_11 takes a plane, then x_11 is playing tennis. From the fact that for all x_11, if x_11 takes a plane, then x_11 is playing tennis, and that for all x_11, if x_11 plays tennis, then x_11 is taking a plane we can infer that for all x_11, x_11 will take a plane if and only if x_11 will play tennis. From the fact that for all x_11, x_11 will take a plane if and only if x_11 will play tennis we can infer that David is taking a plane if and only if David is playing tennis. Finally, from the fact that David is taking a plane if and only if David is playing tennis we can infer that if David plays tennis, then David is taking a plane. Therefore, the answer is yes.