*   <li>keys automatically wrapped in {@code WeakReference}
 *   <li>values automatically wrapped in {@code WeakReference} or {@code SoftReference}
 *   <li>notification of evicted (or otherwise removed) entries
 *   <li>accumulation of cache access statistics
 * </ul>
 *
 * <p>These features are all optional; caches can be created using all or none of them. By default,

        return ImmutableEnumSet.asImmutable(enumSet);
      } else {
        return ImmutableSet.of();
      }
    }
  }

  /**
   * Returns a {@code Collector} that accumulates the input elements into a new {@code ImmutableSet}
   * with an implementation specialized for enums. Unlike {@link ImmutableSet#toImmutableSet}, the

   * specified supplier. The keys and values of the entries are the result of applying the provided
   * mapping functions to the input elements, accumulated in the encounter order of the stream.
   *
   * <p>Example:
   *
   * {@snippet :
   * static final ListMultimap<Character, String> FIRST_LETTER_MULTIMAP =

   * specified supplier. The keys and values of the entries are the result of applying the provided
   * mapping functions to the input elements, accumulated in the encounter order of the stream.
   *
   * <p>Example:
   *
   * {@snippet :
   * static final ListMultimap<Character, String> FIRST_LETTER_MULTIMAP =