* Implementation for the EntryIterator, which is used to build Key and Value iterators.
   *
   * <p>Expiration is only checked on hasNext(), so as to ensure that a next() call never returns
   * null when hasNext() has already been called.
   */
  class EntryIterator implements Iterator<Entry<K, V>> {
    Iterator<Entry<K, Timestamped<V>>> iterator;
    Entry<K, Timestamped<V>> lastEntry;
    Entry<K, Timestamped<V>> nextEntry;

     * lookups, and dead values, which have been evicted but awaiting removal. Non-active values
     * consist strictly of loading values, though during refresh a value may be both active and
     * loading.
     */
    boolean isActive();
  }

  /** Placeholder. Indicates that the value hasn't been set yet. */
  static final ValueReference<Object, Object> UNSET =

     * lookups, and dead values, which have been evicted but awaiting removal. Non-active values
     * consist strictly of loading values, though during refresh a value may be both active and
     * loading.
     */
    boolean isActive();
  }

  /** Placeholder. Indicates that the value hasn't been set yet. */
  static final ValueReference<Object, Object> UNSET =