Consider the following premises. For all y_4, y_4 will make tea if and only if y_4 is an electrician. There is at least one y_4 for which y_4 will not make tea. For all y_4, if y_4 were an electrician, then y_4 is making tea. Can we infer the following from them? If we can, name the inference rule being used: For all y_4, y_4 is an electrician.
 From the fact that for all y_4, y_4 will make tea if and only if y_4 is an electrician we can infer that for all y_4, if y_4 makes tea, then y_4 is an electrician via universal biconditional elimination. From the fact that for all y_4, if y_4 makes tea, then y_4 is an electrician, and that for all y_4, if y_4 were an electrician, then y_4 is making tea we can infer that for all y_4, y_4 will make tea if and only if y_4 is an electrician via universal biconditional introduction. Finally, from the fact that for all y_4, y_4 will make tea if and only if y_4 is an electrician, and that there is at least one y_4 for which y_4 will not make tea we can infer that there is at least one y_4 for which y_4 is not an electrician via existential biconditional elimination, which contradicts that For all y_4, y_4 is an electrician. Therefore, the answer is no.