Consider the following premises. It is snowing or Richard is not a mechanic. Richard is a mechanic if and only if it is snowing. Can we infer the following from them? If we can, name the inference rule being used: The fact that Richard were a mechanic does not imply that it is snowing.
 From the fact that Richard is a mechanic if and only if it is snowing we can infer that if it snows, then Richard is a mechanic via biconditional elimination. Finally, from the fact that it is snowing or Richard is not a mechanic we can infer that if Richard were a mechanic, then it is snowing via rewriting, which contradicts that The fact that Richard were a mechanic does not imply that it is snowing. Therefore, the answer is no.