code: def fold_sfs(s, n):
      
  
      # check inputs
      s = asarray_ndim(s, 1)
      assert s.shape[0] <= n + 1, 'invalid number of chromosomes'
  
      # need to check s has all entries up to n
      if s.shape[0] < n + 1:
          sn = np.zeros(n + 1, dtype=s.dtype)
          sn[:s.shape[0]] = s
          s = sn
  
      # fold
      nf = (n + 1) // 2
      n = nf * 2
      o = s[:nf] + s[nf:n][::-1]
  
      return o
docs: Fold a site frequency spectrum.
  
      Parameters
      ----------
      s : array_like, int, shape (n_chromosomes,)
          Site frequency spectrum
      n : int
          Total number of chromosomes called.
  
      Returns
      -------
      sfs_folded : ndarray, int
          Folded site frequency spectrum