Consider the following premises. There is at least one x_2 for which if x_2 were an office clerk, then x_2 is reading a book. For all x_2, x_2 is an office clerk or x_2 will not read a book. Can we infer the following from them? If we can, name the inference rule being used: There is at least one x_2 for which x_2 is an office clerk if and only if x_2 will read a book.
 From the fact that for all x_2, x_2 is an office clerk or x_2 will not read a book we can infer that for all x_2, if x_2 reads a book, then x_2 is an office clerk via universal rewriting. Finally, from the fact that there is at least one x_2 for which if x_2 were an office clerk, then x_2 is reading a book, and that for all x_2, if x_2 reads a book, then x_2 is an office clerk we can infer that there is at least one x_2 for which x_2 is an office clerk if and only if x_2 will read a book via existential biconditional introduction. Therefore, the answer is yes.