Consider the following premises. There is at least one z_1 for which z_1 is an office clerk if and only if z_1 will write a letter. For all z_1, if z_1 writes a letter, then z_1 is an office clerk. Can we infer the following from them? If we can, name the inference rule being used: There is at least one z_1 for which if z_1 were an office clerk, then z_1 is writing a letter.
We can infer this via the existential biconditional elimination rule. Therefore, the answer is yes.