def _calc_detlam(xx, yy, zz, yx, zx, zy):
    """
    Calculate determinant of symmetric 3x3 matrix

    [[xx,yx,xz],
     [yx,yy,zy],
     [zx,zy,zz]]
     """
    
    return zz * (yy*xx - yx**2) - \
           zy * (zy*xx - zx*yx) + \
           zx * (zy*yx - zx*yy)