`context` variables are hard to understand

[AltGr]

input and output vars are reasonably easy to make sense of, but the third kind, context, is I think a tripping point when learning the language. The tutorial could be improved in that area, but I believe the name "context" is part of the problem here, it requires meta-understanding of the language structure to figure out why it was named that way.

I believe replacing it with something much simpler like optional input could make things much clearer.