static <A extends Set<S>> A addAndRemoveSomeRandomStrings(A set, int nAdd, int nRemove) {
  repeat nAdd {
    set.add(aGlobalID());
  }
  var it = shuffledIterator(cloneList(set));
  while (nRemove-- > 0 && it.hasNext())
    set.remove(it.next());
  ret set;
}