synchronized (mutex) {
        return delegate().containsValue(value);
      }
    }

    @Override
    public boolean containsEntry(@CheckForNull Object key, @CheckForNull Object value) {
      synchronized (mutex) {
        return delegate().containsEntry(key, value);
      }
    }

    @Override
    public Collection<V> get(@ParametricNullness K key) {
      synchronized (mutex) {