return (BiMap<K, V>) super.delegate();
    }

    @Override
    public Set<V> values() {
      synchronized (mutex) {
        if (valueSet == null) {
          valueSet = set(delegate().values(), mutex);
        }
        return valueSet;
      }
    }

    @Override
    @CheckForNull
    public V forcePut(@ParametricNullness K key, @ParametricNullness V value) {