> Unless I'm reading it wrong, when _setevents() is called, the internal
> hooks are rewired to populate the events list, rather than call the
> corresponding TreeBuilder methods. So, yes, there's a TreeBuilder
> somewhere, but it stands unused.

Yes, you *are* reading it wrong. For example, the "start" callback calls self._start_list, which in turn calls self.target.start(), thus calling into the TreeBuilder. That's the thing that constructs the elements that the IncrementalParser collects in its events list.