code: def _expand(self, coeffsin, nmax):
          
          if self.coeffs is None:
              self.rotate(clat=90., clon=0., nrot=nmax)
              falpha = _shtools.SlepianCoeffs(self.coeffs, coeffsin, self.nrot)
          else:
              falpha = _shtools.SlepianCoeffs(self.coeffs, coeffsin, self.nrot)
  
          return SlepianCoeffs(falpha, self)
docs: Determine the Slepian expansion coefficients of a function.