code: def expand(self, flm, nmax=None):
          
          if nmax is None:
              nmax = (self.lmax+1)**2
          elif nmax is not None and nmax > (self.lmax+1)**2:
              raise ValueError(
                  "nmax must be less than or equal to (lmax+1)**2 " +
                  "where lmax is {:s}. Input value is {:s}"
                  .format(repr(self.lmax), repr(nmax))
                  )
  
          coeffsin = flm.to_array(normalization='4pi', csphase=1, lmax=self.lmax)
  
          return self._expand(coeffsin, nmax)
docs: Return the Slepian expansion coefficients of the input function.
  
          Usage
          -----
          s = x.expand(flm, [nmax])
  
          Returns
          -------
          s : SlepianCoeff class instance
              The Slepian expansion coefficients of the input function.
  
          Parameters
          ----------
          flm : SHCoeffs class instance
              The input function to expand in Slepian functions.
          nmax : int, optional, default = (x.lmax+1)**2
              The number of Slepian expansion coefficients to compute.
  
          Description
          -----------
          The global function f is input using its spherical harmonic
          expansion coefficients flm. The expansion coefficients of the function
          f using Slepian functions g is given by
          
          f_alpha = sum_{lm}^{lmax} f_lm g(alpha)_lm