Consider the following premises. For all z, z will take a plane if and only if z is a poet. For all z, if z takes a plane, then z is a poet. For all z, if z were a poet, then z is taking a plane. Can we infer the following from them? If we can, name the inference rule being used: There is at least one z for which if is not true that z is a poet if and only if z will take a plane.
 From the fact that for all z, if z were a poet, then z is taking a plane, and that for all z, if z takes a plane, then z is a poet we can infer that for all z, z is a poet if and only if z will take a plane via universal biconditional introduction. From the fact that for all z, z will take a plane if and only if z is a poet we can infer that for all z, if z were a poet, then z is taking a plane via universal biconditional elimination. From the fact that for all z, z is a poet if and only if z will take a plane we can infer that for all z, if z takes a plane, then z is a poet via universal biconditional elimination. Finally, from the fact that for all z, if z were a poet, then z is taking a plane, and that for all z, if z takes a plane, then z is a poet we can infer that for all z, z is a poet if and only if z will take a plane via universal biconditional introduction, which contradicts that There is at least one z for which if is not true that z is a poet if and only if z will take a plane. Therefore, the answer is no.