Consider the following premises. It is snowing if and only if Robert is sleeping. Robert is sleeping if and only if it is sunny. If Robert sleeps, then it is sunny. Can we infer the following from them? If we can, name the inference rule being used: The fact that it snows does not imply that it is sunny.
 From the fact that Robert is sleeping if and only if it is sunny we can infer that if it is sunny, then Robert is sleeping via biconditional elimination. From the fact that it is snowing if and only if Robert is sleeping we can infer that if it snows, then Robert is sleeping via biconditional elimination. From the fact that if Robert sleeps, then it is sunny, and that if it is sunny, then Robert is sleeping we can infer that Robert is sleeping if and only if it is sunny via biconditional introduction. From the fact that Robert is sleeping if and only if it is sunny we can infer that if Robert sleeps, then it is sunny via biconditional elimination. Finally, from the fact that if it snows, then Robert is sleeping, and that if Robert sleeps, then it is sunny we can infer that if it snows, then it is sunny via transitivity, which contradicts that The fact that it snows does not imply that it is sunny. Therefore, the answer is no.