Consider the following premises. For all z_4, if z_4 makes tea, then z_4 is a musician. For all z_4, if z_4 were a musician, then z_4 is making tea. For all z_4, z_4 will not work. For all z_4, if z_4 works, then z_4 is a musician. For all z_4, if z_4 were a musician, then z_4 is working. Can we infer the following from them? If we can, name the inference rule being used: There is at least one z_4 for which z_4 will make tea.
 From the fact that for all z_4, if z_4 were a musician, then z_4 is working, and that for all z_4, if z_4 works, then z_4 is a musician we can infer that for all z_4, z_4 is a musician if and only if z_4 will work via universal biconditional introduction. From the fact that for all z_4, if z_4 were a musician, then z_4 is making tea, and that for all z_4, if z_4 makes tea, then z_4 is a musician we can infer that for all z_4, z_4 is a musician if and only if z_4 will make tea via universal biconditional introduction. From the fact that for all z_4, z_4 is a musician if and only if z_4 will work, and that for all z_4, z_4 will not work we can infer that for all z_4, z_4 is not a musician via universal biconditional elimination. Finally, from the fact that for all z_4, z_4 is a musician if and only if z_4 will make tea, and that for all z_4, z_4 is not a musician we can infer that for all z_4, z_4 will not make tea via universal biconditional elimination, which contradicts that There is at least one z_4 for which z_4 will make tea. Therefore, the answer is no.