// threshold must be at least 2
// not synchronized
// untested
srecord BigLittleMapPolicy(Class<? extends Map> smallClass, Class<? extends Map> bigClass, int threshold) implements IMapImplementationPolicy {
  bool nullOnEmpty = false;
  
  public <A, B> Map<A, B> makeSingletonMap(A key, B value) {
    ret put(null, key, value);
  }
  
  public <A, B> Map<A, B> put(Map<A, B> map, A key, B value) {
    if (map == null) map = newInstance(smallClass);
    if (l(map) == threshold-1 && !map.containsKey(key))
      map = putAll(newInstance(bigClass), map);
    map.put(key, value);
    ret map;
  }
  
  public <A, B> Map<A, B> remove(Map<A, B> map, O key) {
    if (map == null || !map.containsKey(key)) ret map;
    map.remove(key);
    int n = l(map);
    if (n == threshold-1)
      map = putAll(newInstance(smallClass), map);
    ret nullOnEmpty && n == 0 ? null : map;
  }
}

Began life as a copy of #1031860