Consider the following premises. David is a mechanic if and only if it is snowing. Linda is a janitor if and only if it is snowing. Robert is curious if and only if Linda is a janitor. Robert is not curious. Can we infer the following from them? David is not a mechanic.
 From the fact that Robert is curious if and only if Linda is a janitor, and that Robert is not curious we can infer that Linda is not a janitor. From the fact that Linda is a janitor if and only if it is snowing, and that Linda is not a janitor we can infer that it is not snowing. Finally, from the fact that David is a mechanic if and only if it is snowing, and that it is not snowing we can infer that David is not a mechanic. Therefore, the answer is yes.