Hmm. Like it says on the label, it takes an iterable of points and returns an iterable of triples in order. But as Stavros notes, it's not at all obvious how it does that. You have to reason your way through it.
It's clockwork, and quite clever - and not the way people think (well, except insofar as people build clockwork and this Python in order to do things like this of course). In terms of code understanding this code is not self-documenting in any way. To determine programmer intent, we have to simulate what it does and see why it does that.
It's kind of like a syntactic artifact of a semantic reasoning process, one that we can recover (hopefully!) with careful reasoning. But the original reasoning is gone.