code: @Override
      public void putAll(Map<? extends K, ? extends V> in) {
          if (fast) {
              synchronized (this) {
                  Map<K, V> temp = cloneMap(map);
                  temp.putAll(in);
                  map = temp;
              }
          } else {
              synchronized (map) {
                  map.putAll(in);
              }
          }
      }
docs: Copy all of the mappings from the specified map to this one, replacing
  any mappings with the same keys.
  
  @param in the map whose mappings are to be copied