docs: def vector(p1, p2):
      """Vector from p1 to p2.
      :param p1: coordinates of point p1
      :param p2: coordinates of point p2
      :returns : numpy array with vector coordinates
      """
      return None if len(p1) != len(p2) else np.array([p2[i] - p1[i] for i in xrange(len(p1))])