Consider the following premises. If it is windy, then Sarah is writing a letter. If Sarah writes a letter, then it is windy. It is not sunny. It is sunny if and only if it is windy. Can we infer the following from them? If we can, name the inference rule being used: Sarah is not writing a letter.
 From the fact that it is sunny if and only if it is windy, and that it is not sunny we can infer that it is not windy via biconditional elimination. From the fact that if it is windy, then Sarah is writing a letter, and that if Sarah writes a letter, then it is windy we can infer that it is windy if and only if Sarah is writing a letter via biconditional introduction. Finally, from the fact that it is windy if and only if Sarah is writing a letter, and that it is not windy we can infer that Sarah is not writing a letter via biconditional elimination. Therefore, the answer is yes.