I love the expression node. It’s powerful, it’s quick and it’s easy to use. One of the useful things you can do with it is create your own little colour keyer. Yes, there are a number of keying options, and yes, this is effectively just the Quick Key. This is more of an experiment in learning the mechanics behind one of the most important techniques in compositing.
This is more of an exercise in learning the limitations of blink script than a functional tool, although it does have some useful alterations and allows for a lot of customisation. The end result is a way of using Point Position and Normals renders to relight an image with a choice of point or spotlight. Directional is easily achieved using these same methods but is not currently part of the script. It will likely be added in future updates.