Category samples
 category provides an easy way for you to modularize the definition of a class into groups or categories of related methods. It also gives you an easy way to extend an existing class definition without even having access to the original source code for the class and without having to create a subclass. This is a powerful yet easy concept for you to learn.

By convention, file namedFraction+MathOps.h and t he implement at ion sect ion in a file called Fraction+MathOps.m.