As Paul or someone pointed out, that's a fairly odd thing to do.  It's the first time that use case has been mentioned in this thread.  It's true you've managed to construct something that isn't done by flatten().  I would have had to think a while to see what you meant by the original if you haven't provided the intermediate interpretations.

Of course, it's *really simple* to spell that in a natural way with existing syntax that isn't confusing like yours:

    [x for end in range(4) for x in range(end)]

There is no possible way to construct something that would use the proposed syntax that can't be expressed more naturally with a nested loop... because it's just confusing syntax sugar for exactly that.

Your example looks like some sort of interview quiz question to see if someone knows obscure and unusual syntax.