def normalize_slice(s, size):
    """ Normalize a python slice such that.

    s - slice instance to normalize
    size - the size of original sequence to normalize for

    The returned slice instance has following properties:
      - start, stop, step are all int
      - start < stop
      - step > 1
    """
    start, stop, step = s.indices(size)
    if step < 0:
        step = -step
        start, stop = stop, start
        diff = stop - start
        if diff % step == 0:
            n = diff//step - 1
        else:
            n = diff//step
        start = stop - n*step

        stop += 1

    return slice(start, stop, step)