def METHOD_NAME(self, x):
    """ See :meth:`pints.LogPDF.evaluateS1()`. """
    L = self.__call__(x)
    self._x_minus_mu = x - self._mean
    # derivative wrt x: see https://stats.stackexchange.com/questions/27436/how-to-take-derivative-of-multivariate-normal-density # noqa
    dL = -np.matmul(self._sigma_inv, self._x_minus_mu)
    return L, dL