r"""The flux network for the reaction from
        A=[0,...,a] => B=[b,...,M].

        Parameters
        ----------
        a : int
            State index
        b : int
            State index

        Returns
        -------
        flux : (M, M) ndarray
            Matrix of flux values between pairs of states.

def flux(self, a, b):
        r"""The flux network for the reaction from
        A=[0,...,a] => B=[b,...,M].

        Parameters
        ----------
        a : int
            State index
        b : int
            State index

        Returns
        -------
        flux : (M, M) ndarray
            Matrix of flux values between pairs of states.

        """
        # if b<=a:
        # raise ValueError("State index b has to be strictly larger than state index a")
        qminus = self.committor_backward(a, b)
        qplus = self.committor_forward(a, b)
        P = self.transition_matrix()
        pi = self.stationary_distribution()

        flux = pi[:, np.newaxis] * qminus[:, np.newaxis] * P * qplus[np.newaxis, :]
        ind = np.diag_indices(P.shape[0])
        flux[ind] = 0.0
        return flux