Consider the following premises. Richard is working if and only if it is sunny. Richard is working if and only if it is sunny. Can we infer the following from them? If we can, name the inference rule being used: If is not true that it is sunny if and only if Richard is working.
 From the fact that Richard is working if and only if it is sunny we can infer that if Richard works, then it is sunny via biconditional elimination. From the fact that Richard is working if and only if it is sunny we can infer that if it is sunny, then Richard is working via biconditional elimination. Finally, from the fact that if it is sunny, then Richard is working, and that if Richard works, then it is sunny we can infer that it is sunny if and only if Richard is working via biconditional introduction, which contradicts that If is not true that it is sunny if and only if Richard is working. Therefore, the answer is no.