Extending Schemes

[Checkmate50]

I find myself occasionally wanting to extend a coordinate scheme with a new type that doesn't really fit in the core library. For example, when writing microfacet, I really want the type halfdir is direction to keep track of my half-directions and make sure I'm calling the updating function.

I'm imagining something like: extend cart3 with type halfdir is direction;. This sort of command could lead to spaghetti code if misused, so maybe highlighting the extend keyword red or preventing it from being imported or something makes sense.