code: def patterson_fst(aca, acb):
      
  
      from allel.stats.admixture import patterson_f2, h_hat
      num = patterson_f2(aca, acb)
      den = num + h_hat(aca) + h_hat(acb)
  
      return num, den
docs: Estimator of differentiation between populations A and B based on the
      F2 parameter.
  
      Parameters
      ----------
      aca : array_like, int, shape (n_variants, 2)
          Allele counts for population A.
      acb : array_like, int, shape (n_variants, 2)
          Allele counts for population B.
  
      Returns
      -------
      num : ndarray, shape (n_variants,), float
          Numerator.
      den : ndarray, shape (n_variants,), float
          Denominator.
  
      Notes
      -----
      See Patterson (2012), Appendix A.
  
      TODO check if this is  numerically equivalent to Hudson's estimator.