Consider the following premises. Richard is writing a letter or Richard is not driving a car. For all x, x will drive a car if and only if x will write a letter. Can we infer the following from them? If we can, name the inference rule being used: Richard is driving a car if and only if Richard is writing a letter.
 From the fact that for all x, x will drive a car if and only if x will write a letter we can infer that for all x, if x writes a letter, then x is driving a car via universal biconditional elimination. From the fact that for all x, if x writes a letter, then x is driving a car we can infer that if Richard writes a letter, then Richard is driving a car via universal instantiation. From the fact that Richard is writing a letter or Richard is not driving a car we can infer that if Richard drives a car, then Richard is writing a letter via rewriting. Finally, from the fact that if Richard drives a car, then Richard is writing a letter, and that if Richard writes a letter, then Richard is driving a car we can infer that Richard is driving a car if and only if Richard is writing a letter via biconditional introduction. Therefore, the answer is yes.