Cool. It's also a lot faster than my version.

Yes. But unfortunately, fatally flawed.

I should never try to assess the inherent complexity of a task when approaching my personal shutdown time :) Nor take example solutions as sufficient tests.

That said, the fix to the approach doesn't seem to be too complicated, though I haven't coded it yet, so that thought might come back to bite me. Maybe I'll get to it once I cleared the non-optional tasks of my day.