Sometimes, we would like to add methods to an existing class. We already know how to do this; we just need to go to its Swift source file and add a new method within the class body. However, sometimes, we cannot access the source code for the class, or it isn't convenient to make changes to it. A typical example of this situation is a class, struct, or any other type that is part of the standard language elements. For example, we might want to add a method that we can call in any Int value to initialize either a 2D or 3D point with all its elements set to the Int value.
The following lines declare a simple Point2D class that represents a mutable 2D point with the x and y elements. The class conforms to the CustomStringConvertible protocol; therefore, it declares a description computed property that returns a string representation for the 2D point:
public class Point2D: CustomStringConvertible {
public var x: Int
public var y: Int
public var valuesAsDescription...
