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).

Thu Sep 14 08:45:18 EDT 1995