static Set<S> joinCISets(Collection<S>... l) {
  Set<S> set = ciSet();
  for (Collection<S> o : l)
    if (o != null)
      set.addAll(o);
  ret set;
}

static Set<S> joinCISets(Iterable<? extends Cl<S>> l) {
  Set<S> set = ciSet();
  if (l != null) for (Collection<S> o : l)
    if (o != null)
      set.addAll(o);
  ret set;
}

Began life as a copy of #1003110