## ABSTRACT

As we have seen above, the notion of set is necessary to construct composite mathematical objects from more elementary ones [15]. For example, a rational number is actually an ordered set of two natural numbers, a real number is actually an inﬁnite set of rational numbers, a point in the Cartesian plane is an ordered set of two real numbers, a line in analytic geometry is an inﬁnite set of points, and so on.