@phobson Can you comment on this?  I would avoid using the word collection as we _have_ a collection class and the same shadowing could be nasty.

I would say something like
```
Converts a non-empty list or ndarray into a list of iterable objects such that

    for x in _reshape_2D(X):
        for y in x:
            foo(y)

works correctly.
```

As this is a 'private' function this documentation is for other devs.