Consider the following premises. For all x_9, if x_9 were a sailor, then x_9 is an electrician. There is at least one x_9 for which x_9 is a sailor if and only if x_9 will play tennis. For all x_9, if x_9 were an electrician, then x_9 is playing tennis. Can we infer the following from them? If we can, name the inference rule being used: For all x_9, if is not true that x_9 is a sailor if and only if x_9 will play tennis.
 From the fact that there is at least one x_9 for which x_9 is a sailor if and only if x_9 will play tennis we can infer that there is at least one x_9 for which if x_9 plays tennis, then x_9 is a sailor via existential biconditional elimination. From the fact that for all x_9, if x_9 were a sailor, then x_9 is an electrician, and that for all x_9, if x_9 were an electrician, then x_9 is playing tennis we can infer that for all x_9, if x_9 were a sailor, then x_9 is playing tennis via universal transitivity. Finally, from the fact that for all x_9, if x_9 were a sailor, then x_9 is playing tennis, and that there is at least one x_9 for which if x_9 plays tennis, then x_9 is a sailor we can infer that there is at least one x_9 for which x_9 is a sailor if and only if x_9 will play tennis via existential biconditional introduction, which contradicts that For all x_9, if is not true that x_9 is a sailor if and only if x_9 will play tennis. Therefore, the answer is no.