N.diff2(i,j)

        Mixed second derivative. Differentiates wrt both indices.

def diff2(self, i, j):
        """
        N.diff2(i,j)

        Mixed second derivative. Differentiates wrt both indices.
        """

        old_val = copy(self[j])

        if not self.stochastic_indices[i][
                0] in self.stochastic_indices[j][0].moral_neighbors:
            return 0.

        def diff_for_diff(val):
            self[j] = val
            return self.diff(i)

        d = derivative(
            func=diff_for_diff,
            x0=old_val,
            dx=self.eps[j],
            n=1,
            order=self.diff_order)

        self[j] = old_val
        return d