Consider the following premises. Thomas is playing squash if and only if it is sunny. Thomas is playing squash. Can we infer the following from them? If we can, name the inference rule being used: It is sunny or Robert is writing a letter.
 From the fact that Thomas is playing squash if and only if it is sunny we can infer that if Thomas plays squash, then it is sunny via biconditional elimination. From the fact that if Thomas plays squash, then it is sunny, and that Thomas is playing squash we can infer that it is sunny via modus ponens. Finally, from the fact that it is sunny we can infer that it is sunny or Robert is writing a letter via addition. Therefore, the answer is yes.