code: def _coupling_matrix(self, lmax, nwin=None, weights=None):
          
          if nwin is None:
              nwin = self.nwin
  
          if weights is None:
              weights = self.weights
  
          if weights is None:
              return _shtools.SHMTCouplingMatrix(lmax, self.tapers**2, k=nwin)
          else:
              return _shtools.SHMTCouplingMatrix(lmax, self.tapers**2, k=nwin,
                                                 taper_wt=self.weights)
docs: Return the coupling matrix of the first nwin tapers.