code: def _coupling_matrix(self, lmax, nwin=None, weights=None):
          
          if nwin is None:
              nwin = self.nwin
  
          if weights is None:
              weights = self.weights
  
          tapers_power = _np.zeros((self.lwin+1, nwin))
          for i in range(nwin):
              tapers_power[:, i] = _spectrum(self.to_array(i),
                                             normalization='4pi',
                                             convention='power', unit='per_l')
  
          if weights is None:
              return _shtools.SHMTCouplingMatrix(lmax, tapers_power, k=nwin)
          else:
              return _shtools.SHMTCouplingMatrix(lmax, tapers_power, k=nwin,
                                                 taper_wt=self.weights)
docs: Return the coupling matrix of the first nwin tapers.