* a bucket in another map.
   */
  boolean isReusable() {
    return true;
  }

  static class NonTerminalImmutableMapEntry<K, V> extends ImmutableMapEntry<K, V> {
    /*
     * Yes, we sometimes set nextInKeyBucket to null, even for this "non-terminal" entry. We don't
     * do that with a plain NonTerminalImmutableMapEntry, but we do it with the BiMap-specific