def METHOD_NAME(reports, active_subset):
  n_tranches = len(reports)
  n_cosets = len(reports[0])
  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)
  return len(total)