def internal_cross(u, v):
    """Returns the cross product of two vectors. Should be identical to the
    output of numpy.cross(u, v).
    """
    return(u[1]*v[2] - v[1]*u[2],
           u[2]*v[0] - v[2]*u[0],
           u[0]*v[1] - v[0]*u[1])