Consider the following premises. For all x_3, x_3 is an artist or x_3 will not take a plane. There is at least one x_3 for which if x_3 were an artist, then x_3 is taking a plane. 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 an artist if and only if x_3 will take a plane.
 From the fact that for all x_3, x_3 is an artist or x_3 will not take a plane we can infer that for all x_3, if x_3 takes a plane, then x_3 is an artist via universal rewriting. Finally, from the fact that there is at least one x_3 for which if x_3 were an artist, then x_3 is taking a plane, and that for all x_3, if x_3 takes a plane, then x_3 is an artist we can infer that there is at least one x_3 for which x_3 is an artist if and only if x_3 will take a plane via existential biconditional introduction. Therefore, the answer is yes.