I love this, but I'm a bit surprised that you do not include the ability to "unexpound" an "expounded" term. Is that intentional?
If I were reading a technical text, I would definitely end up reading most paragraphs at least twice. It would make no sense to keep the expounded terms in the second time; I'd be tempted to hide them back as soon as I was finished with them the first time.
It's because, once clicked, the new text should become part of the old, and that's it. Presumably you've already read it, and I don't want to make the viewer have to re-collapse the links every time.
Your use case makes sense, though, which is why the feature was included. Maybe I should mention it in the README.
I think collapsing would also be useful when all you need is a quick reminder, not a full explanation. Like "What's that again? [click to expand] Oh that's right [click to collapse]". That's easier than finding the place to skip to.
If I were reading a technical text, I would definitely end up reading most paragraphs at least twice. It would make no sense to keep the expounded terms in the second time; I'd be tempted to hide them back as soon as I was finished with them the first time.