srecord FixedMapClass(Class<? extends Map> clazz) implements IMapImplementationPolicy {
  bool sync = true, 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(clazz);
    if (sync)
      syncPut(map, key, value);
    else
      map.put(key, value);
    ret map;
  }
  
  public <A, B> Map<A, B> remove(Map<A, B> map, O key) {
    if (map == null) null;
    if (sync)
      syncRemove(map, (A) key);
    else
      map.remove(key);
    ret nullOnEmpty && empty(map) ? null : map;
  }
}

Began life as a copy of #1031859