Consider the following premises. For all x_7, x_7 is a sailor if and only if x_7 is a politician. If John were a sailor, then it is sunny. It is not sunny. Can we infer the following from them? If we can, name the inference rule being used: John is not a politician.
 From the fact that for all x_7, x_7 is a sailor if and only if x_7 is a politician we can infer that John is a sailor if and only if John is a politician via universal instantiation. From the fact that if John were a sailor, then it is sunny, and that it is not sunny we can infer that John is not a sailor via modus tollens. Finally, from the fact that John is a sailor if and only if John is a politician, and that John is not a sailor we can infer that John is not a politician via biconditional elimination. Therefore, the answer is yes.