Consider the following premises. For all x_7, x_7 is sad if and only if x_7 is a taxi driver. For all x_7, x_7 is not a taxi driver. For all x_7, x_7 is sad or x_7 is happy. Can we infer the following from them? There is at least one x_7 for which x_7 is not happy.
 From the fact that for all x_7, x_7 is sad if and only if x_7 is a taxi driver we can infer that for all x_7, if x_7 were sad, then x_7 is a taxi driver. From the fact that for all x_7, if x_7 were sad, then x_7 is a taxi driver, and that for all x_7, x_7 is not a taxi driver we can infer that for all x_7, x_7 is not sad. Finally, from the fact that for all x_7, x_7 is sad or x_7 is happy, and that for all x_7, x_7 is not sad we can infer that for all x_7, x_7 is happy, which contradicts that There is at least one x_7 for which x_7 is not happy. Therefore, the answer is no.