Also see Operator Definitions for newer supplementary material. The modern method treats definitions as more than a matter of appearance.
The definition facility allows one to develop new notations in the form of templates which can be invoked when entering text. This feature provides a flexible way to introduce new notation. For instance, we might have defined a function, abs(x), which computes the absolute value of a number, but we might like to display it in the text as |x|. This can be accomplished by defining a template. The format is |<x:number>| == abs(<x>), where the left--hand side of == is the template and the right--hand side is the value (the angle brackets are used to designate the parameter).