Consider the following premises. For all x, x is sad if and only if x is a server. For all x, x is sad if and only if x is a server. Can we infer the following from them? If we can, name the inference rule being used: For all x, x is a server if and only if x is sad.
 From the fact that for all x, x is sad if and only if x is a server we can infer that for all x, if x were sad, then x is a server via universal biconditional elimination. From the fact that for all x, x is sad if and only if x is a server we can infer that for all x, if x were a server, then x is sad via universal biconditional elimination. Finally, from the fact that for all x, if x were a server, then x is sad, and that for all x, if x were sad, then x is a server we can infer that for all x, x is a server if and only if x is sad via universal biconditional introduction. Therefore, the answer is yes.