def ccw(a, b, c):
    """Tests whether the turn formed by a, b, and c is CCW

    Returns:
        > 0 for c left of the line through a and b (ccw)
        = 0 for b on the line
        < 0 for b right of the line (not ccw)
    """
    return (b[0] - a[0]) * (c[1] - a[1]) - (c[0] - a[0]) * (b[1] - a[1])
