Subtract: v - w.  Assume the first arg v is a Vec2.
    The second arg w can be a vec2 or a number.

def sub_v2(v, w):
    """Subtract: v - w.  Assume the first arg v is a Vec2.
    The second arg w can be a vec2 or a number.
    """

    if type(w) is float or type(w) is int:
        return Vec2(v.x - w, v.y - w)
    else:
        return Vec2(v.x - w.x, v.y - w.y)