code: def joint_sfs_folded(ac1, ac2, n1=None, n2=None):
      
  
      # check inputs
      ac1, n1 = _check_ac_n(ac1, n1)
      ac2, n2 = _check_ac_n(ac2, n2)
  
      # compute minor allele counts
      mac1 = np.amin(ac1, axis=1)
      mac2 = np.amin(ac2, axis=1)
  
      # compute site frequency spectrum
      x = n1//2 + 1
      y = n2//2 + 1
      tmp = (mac1 * y + mac2).astype(int, copy=False)
      s = np.bincount(tmp)
      s.resize(x, y)
      return s
docs: Compute the joint folded site frequency spectrum between two
      populations.
  
      Parameters
      ----------
      ac1 : array_like, int, shape (n_variants, 2)
          Allele counts for the first population.
      ac2 : array_like, int, shape (n_variants, 2)
          Allele counts for the second population.
      n1, n2 : int, optional
          The total number of chromosomes called in each population.
  
      Returns
      -------
      joint_sfs_folded : ndarray, int, shape (n1//2 + 1, n2//2 + 1)
          Array where the (i, j)th element is the number of variant sites with a
          minor allele count of i in the first population and j in the second
          population.