def centre_size_to_sides(centre, size):
    """Returns a 4-tuple of the relevant ordinates of the left,
    right, top and bottom sides of the described rectangle"""
    (x, y) = centre.to_pair()
    (half_width, half_height) = (size.scale(0.5)).to_pair()
    left = x - half_width
    right = x + half_width
    top = y - half_height
    bottom = y + half_height
    return (left, right, top, bottom)