Vector from p1 to p2.
    :param p1: coordinates of point p1
    :param p2: coordinates of point p2
    :returns : numpy array with vector coordinates

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))])