static Set<S> thingsOnScreen_newestFirst_lastSet;
static new Map<S, Long> thingsOnScreen_newestFirst_latestAppearance;

static L<S> thingsOnScreen_newestFirst() {
  Set<S> things = thingsOnScreen();
  lock thingsOnScreen_newestFirst_lock;
  for (S s : setMinusSet(things, thingsOnScreen_newestFirst_lastSet))
    thingsOnScreen_newestFirst_latestAppearance.put(s, sysNow());
  thingsOnScreen_newestFirst_lastSet = things;
  ret sortedByMapLookupDesc(things, thingsOnScreen_newestFirst_latestAppearance);
}