def _compute_diag_H(self, t, index, update_derivatives=False, stationary=False):
"""<STR_LIT>"""<EOL>pass<EOL>
Helper function for computing H for the diagonal only.
        :param t: time input.
        :type t: array
        :param index: first output indices
        :type index: array of int.
        :param index: second output indices
        :type index: array of int.
        :param update_derivatives: whether or not to update the derivative portions (default False).
        :type update_derivatives: bool
        :param stationary: whether to compute the stationary version of the covariance (default False).
        :type stationary: bool