code: def condensed_coords_between(pop1, pop2, n):
      
  
      return [condensed_coords(i, j, n)
              for i, j in itertools.product(sorted(pop1), sorted(pop2))]
docs: Return indices into a condensed distance matrix for all pairwise
      comparisons between two populations.
  
      Parameters
      ----------
      pop1 : array_like, int
          Indices of samples or haplotypes within the first population.
      pop2 : array_like, int
          Indices of samples or haplotypes within the second population.
      n : int
          Size of the square matrix (length of first or second dimension).
  
      Returns
      -------
      indices : ndarray, int