We're only dealing with "data source" arrays at this point (i.e. **not** instances of `_Aggregation`), so there's no recursion possible. Data source arrays (such as NumpyArrayAdapter) can always provide their contents as a concrete array without using an Engine.

If you're thinking something along the lines of "Oh... really?! That's fragile isn't it?!", then I'd have to agree. :wink: Perhaps now is the time to do something about this?