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)
