def METHOD_NAME(reports, active_subset):
  """Once the tranches are aligned
         tranche0 tranche1 tranche2
   coset0    A       C        E
   coset1    B       D        F
   The total number of datasets is union (A,....,F)
   This should also be equal to union (A,C,E) + union (B,D,F)
   Not true prior to alignment.
  """
  n_tranches = len(reports)
  n_cosets = len(reports[0])
  coset_totals = [set() for c in range(n_cosets)]
  total = set()
  for itranche in range(n_tranches):
    if not active_subset[itranche]: continue
    for icoset in range(n_cosets):
      cut = set(reports[itranche][icoset])
      total = total.union(cut)
      coset_totals[icoset] = coset_totals[icoset].union(cut)
  test = 0
  for icoset in range(n_cosets):
    test += len(coset_totals[icoset])
  print("Aligned total is",test,"grand total is",len(total), "delta",test-len(total))
  return test-len(total)