Hmm.  OK, so it is just Callable that is the odd man out.

But in that case, shouldn't the pattern be adopted by the other modules that define abcs as well?  And if the distinction isn't sharp enough in those other modules to justify that, then I question whether it is a good idea to special-case collections.  The docs already make the distinction pretty clear.

My "flat is better than nested" alarm is going off here :)