Iterator<? extends T> iterator) {
    if (iterator instanceof PeekingImpl) {
      // Safe to cast <? extends T> to <T> because PeekingImpl only uses T
      // covariantly (and cannot be subclassed to add non-covariant uses).
      @SuppressWarnings("unchecked")
      PeekingImpl<T> peeking = (PeekingImpl<T>) iterator;
      return peeking;
    }
    return new PeekingImpl<>(iterator);
  }

 * href="https://github.com/google/guava/wiki/UsingAndAvoidingNullExplained#optional">using {@code
 * Optional}</a>.
 *
 * @param <T> the type of instance that can be contained. {@code Optional} is naturally covariant on
 *     this type, so it is safe to cast an {@code Optional<T>} to {@code Optional<S>} for any
 *     supertype {@code S} of {@code T}.
 * @author Kurt Alfred Kluever
 * @author Kevin Bourrillion
 * @since 10.0
 */

        checkNotNull(cell.getRowKey(), "row");
        checkNotNull(cell.getColumnKey(), "column");
        checkNotNull(cell.getValue(), "value");
        @SuppressWarnings("unchecked") // all supported methods are covariant
        Cell<R, C, V> immutableCell = (Cell<R, C, V>) cell;
        cells.add(immutableCell);
      } else {
        put(cell.getRowKey(), cell.getColumnKey(), cell.getValue());
      }
      return this;

   */
  public static <K, V> ImmutableBiMap<K, V> copyOf(
      Iterable<? extends Entry<? extends K, ? extends V>> entries) {
    @SuppressWarnings("unchecked") // we'll only be using getKey and getValue, which are covariant
    Entry<K, V>[] entryArray = (Entry<K, V>[]) Iterables.toArray(entries, EMPTY_ENTRY_ARRAY);
    switch (entryArray.length) {
      case 0:
        return of();
      case 1:

   */
  public static <K, V> ImmutableMap<K, V> copyOf(
      Iterable<? extends Entry<? extends K, ? extends V>> entries) {
    @SuppressWarnings("unchecked") // we'll only be using getKey and getValue, which are covariant
    Entry<K, V>[] entryArray = (Entry<K, V>[]) Iterables.toArray(entries, EMPTY_ENTRY_ARRAY);
    switch (entryArray.length) {
      case 0:
        return of();
      case 1: