Given start, stop, and stop, calculate the number of elements
    of this slice.

def _get_dim_size(start, stop, step):
    """Given start, stop, and stop, calculate the number of elements
    of this slice."""
    assert step != 0
    if step > 0:
        assert start < stop
        dim_size = (stop - start - 1) // step + 1
    else:
        assert stop < start
        dim_size = (start - stop - 1) // (-step) + 1
    return dim_size