[some-mthfka]

You are absolutely on point, yes. The building blocks themselves are what the users will extend upon and use. That plays a very big role in composition and reuse. There will also be configurations and contexts (which are really quite simple mechanisms, really) that will factor into this, too (for the purposes of customization so that the users don't have to modify the original code to change some behavior or slot). Of course, prototype OO itself has a key agency here.

I also like to think about this in terms of "building blocks", not just an exposition of API. So, Emacs has the notion of a buffer for its building block (the only one, I believe). Cells and lenses will be building blocks.