<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def angle(u, v, w=None):
 """Returns the angle in radians between two vectors. """

    if w is not None:
        u = u - v
        v = w - v
    c = np.dot(u, v) / norm(u) / norm(v)
    return np.arccos(np.clip(c, -1, 1))