Consider the following premises. Robert is a taxi driver. If it is windy, then Robert is not a taxi driver. John is a janitor if and only if it is windy. Can we infer the following from them? John is not a janitor.
 From the fact that John is a janitor if and only if it is windy we can infer that if John were a janitor, then it is windy. From the fact that if it is windy, then Robert is not a taxi driver, and that Robert is a taxi driver we can infer that it is not windy. Finally, from the fact that if John were a janitor, then it is windy, and that it is not windy we can infer that John is not a janitor. Therefore, the answer is yes.