def gradients_XX(self, dL_dK, X, X2, cov=True):
raise NotImplementedError("<STR_LIT>")<EOL>
.. math::

    \\frac{\partial^2 L}{\partial X\partial X_2} = \\frac{\partial L}{\partial K}\\frac{\partial^2 K}{\partial X\partial X_2}