return this;
    }

    @Override
    public void setPreviousInWriteQueue(ReferenceEntry<Object, Object> previous) {}
  }

  abstract static class AbstractReferenceEntry<K, V> implements ReferenceEntry<K, V> {
    @Override
    public ValueReference<K, V> getValueReference() {
      throw new UnsupportedOperationException();
    }

    @Override