static Set similarEmptySet(Iterable m) {
  if (m instanceof TreeSet) ret new TreeSet(((TreeSet) m).comparator());
  if (m instanceof LinkedHashSet) ret new LinkedHashSet;
  
  ret new HashSet;
}

static Set similarEmptySet(Map m) {
  if (m instanceof TreeMap) ret new TreeSet(((TreeMap) m).comparator());
  if (m instanceof LinkedHashMap) ret new LinkedHashSet;
  
  ret new HashSet;
}

Began life as a copy of #1004367