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