Consider the following premises. For all x_11, if x_11 were a taxi driver, then x_11 is poor. For all x_11, if x_11 were poor, then x_11 is a taxi driver. For all x_11, x_11 will not play tennis or x_11 is sad. For all x_11, x_11 will play tennis or x_11 is poor. For all x_11, x_11 is not happy. For all x_11, x_11 is happy if and only if x_11 is a taxi driver. Can we infer the following from them? If we can, name the inference rule being used: For all x_11, x_11 is sad.
 From the fact that for all x_11, if x_11 were a taxi driver, then x_11 is poor, and that for all x_11, if x_11 were poor, then x_11 is a taxi driver we can infer that for all x_11, x_11 is a taxi driver if and only if x_11 is poor via universal biconditional introduction. From the fact that for all x_11, x_11 is happy if and only if x_11 is a taxi driver, and that for all x_11, x_11 is not happy we can infer that for all x_11, x_11 is not a taxi driver via universal biconditional elimination. From the fact that for all x_11, x_11 is a taxi driver if and only if x_11 is poor, and that for all x_11, x_11 is not a taxi driver we can infer that for all x_11, x_11 is not poor via universal biconditional elimination. From the fact that for all x_11, x_11 will play tennis or x_11 is poor, and that for all x_11, x_11 will not play tennis or x_11 is sad we can infer that for all x_11, x_11 is poor or x_11 is sad via universal resolution. Finally, from the fact that for all x_11, x_11 is poor or x_11 is sad, and that for all x_11, x_11 is not poor we can infer that for all x_11, x_11 is sad via universal disjunctive syllogism. Therefore, the answer is yes.