sclass LatestAppearance<A> {
  Set<A> lastSet;
  new Map<A, Long> latestAppearance;

  L<A> recordAndSort(Set<A> set) {
    for (S s : setMinusSet(set, lastSet))
      latestAppearance.put(s, sysNow());
    lastSet = set;
    ret sortedByMapLookupDesc(set, latestAppearance);
  }
}