/**
 * {@code keySet()} implementation for {@link ImmutableMap}.
 *
 * @author Jesse Wilson
 * @author Kevin Bourrillion
 */
@GwtCompatible
final class ImmutableMapKeySet<K, V> extends IndexedImmutableSet<K> {
  private final ImmutableMap<K, V> map;

  ImmutableMapKeySet(ImmutableMap<K, V> map) {
    this.map = map;
  }

  @Override
  public int size() {
    return map.size();
  }

  @Override

  }

  @Override
  ImmutableSet<K> createKeySet() {
    return new KeySet<>(this);
  }

  private static final class KeySet<K> extends IndexedImmutableSet<K> {
    private final RegularImmutableMap<K, ?> map;

    KeySet(RegularImmutableMap<K, ?> map) {
      this.map = map;
    }

    @Override
    K get(int index) {