static Map similarEmptyMap(Map m) {
  if (m instanceof TreeMap) ret new TreeMap(((TreeMap) m).comparator());
  if (m instanceof LinkedHashMap) ret new LinkedHashMap;
  
  // default to a hash map
  ret new HashMap;
}

static Map similarEmptyMap(Iterable m) {
  if (m instanceof TreeSet) ret new TreeMap(((TreeSet) m).comparator());
  if (m instanceof LinkedHashSet) ret new LinkedHashMap;
  
  ret new HashMap;
}