Consider the following premises. For all y, if y makes tea, then y is a lawyer. For all y, if y were a server, then y is making tea. There is at least one y for which y is a lawyer if and only if y is a server. Can we infer the following from them? If we can, name the inference rule being used: There is at least one y for which y is a server if and only if y is a lawyer.
 From the fact that for all y, if y were a server, then y is making tea, and that for all y, if y makes tea, then y is a lawyer we can infer that for all y, if y were a server, then y is a lawyer via universal transitivity. From the fact that there is at least one y for which y is a lawyer if and only if y is a server we can infer that there is at least one y for which if y were a lawyer, then y is a server via existential biconditional elimination. Finally, from the fact that for all y, if y were a server, then y is a lawyer, and that there is at least one y for which if y were a lawyer, then y is a server we can infer that there is at least one y for which y is a server if and only if y is a lawyer via existential biconditional introduction. Therefore, the answer is yes.