abstract Equivalence<Object> defaultEquivalence();
  }

  /**
   * A helper object for operating on {@link InternalEntry} instances in a type-safe and efficient
   * manner.
   *
   * <p>For each of the four combinations of strong/weak key and strong/weak value, there are
   * corresponding {@link InternalEntry}, {@link Segment}, and {@link InternalEntryHelper}
   * implementations.
   *