* consistency, and because the return type conveys the immutability guarantee.
   *
   * <p><b>Performance note:</b> the instance returned is a singleton.
   */
  @SuppressWarnings({"unchecked"}) // fully variant implementation (never actually produces any Es)
  public static <E> ImmutableSet<E> of() {
    return (ImmutableSet<E>) RegularImmutableSet.EMPTY;
  }

  /**

    private ImmutableMap<K, V> build(boolean throwIfDuplicateKeys) {
      if (throwIfDuplicateKeys && duplicateKey != null) {
        throw duplicateKey.exception();
      }
      /*
       * If entries is full, then this implementation may end up using the entries array
       * directly and writing over the entry objects with non-terminal entries, but this is

   * NavigableSet#tailSet(Object, boolean) tailSet()}, and {@link NavigableSet#headSet(Object,
   * boolean) headSet()}) to actually construct the view. Consult these methods for a full
   * description of the returned view's behavior.
   *
   * <p><b>Warning:</b> {@code Range}s always represent a range of values using the values' natural

   * NavigableMap#tailMap(Object, boolean) tailMap()}, and {@link NavigableMap#headMap(Object,
   * boolean) headMap()}) to actually construct the view. Consult these methods for a full
   * description of the returned view's behavior.
   *
   * <p><b>Warning:</b> {@code Range}s always represent a range of values using the values' natural