When defining your own notations it is critically important to use the notation templates. The notation templates are contained in the notation palette shown below.
The reason for this restriction is that the templates pasted into a notebook contain essential tag boxes embedded in the correct way. These embedded tag boxes allow Notation , Symbolize and InfixNotation to properly group and parse the new notation being defined. Further, they allow these functions to get the styling information of the new notation so that the output is formatted with the same spacing, sizes, adjustments, etc. as the input.
A blank notation template looks like the following.
Advanced note: You can circumvent using the notation palette by wrapping the corresponding literal box structures in a NotationBoxTag.
In Mathematica it is possible to create input aliases for any notation or expression you want. The normal interface to these input aliases is rather cumbersome and so the notation package contains the function AddInputAlias to facilitate adding input aliases to the current notebook. Each of the common templates has an alias associated with it for easy entry. For instance typing notation in an input cell will enter the notation template .
Add an input alias for apply statements to the current notebook.
Now that we have created this alias we can enter "apply" templates by simply typing apply.