 
 <prompt> Wetin you no fit call the formal group? Given the previous question, write a context wey contain the answer. E fit be 1 - 20 sentences. Context: <response>: The second way wey dem take solve the problem na based on the idea of class, and e define set as class wey dey belong to other classes, while proper class na class wey no dey belong to other classes. Under the Zermelo-Fraenkel approach, the axioms dey prevent the construction of a set wey get all sets wey no dey belong to themselves. But under the von Neumann approach, the class of all sets wey no dey belong to themselves fit be constructed, but e go be proper class and no be set. 