Covariance-derived term to construct expectations. See Rasmussen & Williams.
        
        Parameters
        ----------
        L : np.ndarray
            Cholesky triangular

        Returns
        ----------
        np.ndarray (alpha)

def _alpha(self, L):
        """ Covariance-derived term to construct expectations. See Rasmussen & Williams.
        
        Parameters
        ----------
        L : np.ndarray
            Cholesky triangular

        Returns
        ----------
        np.ndarray (alpha)
        """     
        return la.cho_solve((L.T, True), la.cho_solve((L, True), np.transpose(self.data)))