self.put(entry.getKey(), entry.getValue());
    }
  }

  static class KeySet<K extends @Nullable Object, V extends @Nullable Object>
      extends Sets.ImprovedAbstractSet<K> {
    @Weak final Map<K, V> map;

    KeySet(Map<K, V> map) {
      this.map = checkNotNull(map);
    }

    Map<K, V> map() {
      return map;
    }

    @Override