Consider the following premises. For all x_11, x_11 will write a letter if and only if x_11 is a server. For all x_11, x_11 is a server if and only if x_11 will write a letter. Can we infer the following from them? If Mary writes a letter, then Mary is a server.
 From the fact that for all x_11, x_11 is a server if and only if x_11 will write a letter we can infer that for all x_11, if x_11 writes a letter, then x_11 is a server. From the fact that for all x_11, x_11 will write a letter if and only if x_11 is a server we can infer that for all x_11, if x_11 were a server, then x_11 is writing a letter. From the fact that for all x_11, if x_11 were a server, then x_11 is writing a letter, and that for all x_11, if x_11 writes a letter, then x_11 is a server we can infer that for all x_11, x_11 is a server if and only if x_11 will write a letter. From the fact that for all x_11, x_11 is a server if and only if x_11 will write a letter we can infer that Mary is a server if and only if Mary is writing a letter. Finally, from the fact that Mary is a server if and only if Mary is writing a letter we can infer that if Mary writes a letter, then Mary is a server. Therefore, the answer is yes.