code: def null_beta(self):
          
          ETBE = self._ETBE
          yTBX = self._yTBX
  
          A = sum(i.XTBX for i in ETBE)
          b = sum(yTBX)
          return rsolve(A, b)
docs: Optimal 𝜷 according to the marginal likelihood.
  
          It is compute by solving the equation ::
  
              (XᵀBX)𝜷 = XᵀB𝐲.
  
          Returns
          -------
          beta : ndarray
              Optimal 𝜷.