}

  @GwtIncompatible // NavigableSet
  static <E extends @Nullable Object> NavigableSet<E> navigableSet(
      NavigableSet<E> navigableSet, @Nullable Object mutex) {
    return new SynchronizedNavigableSet<>(navigableSet, mutex);
  }

  @GwtIncompatible // NavigableSet
  static <E extends @Nullable Object> NavigableSet<E> navigableSet(NavigableSet<E> navigableSet) {
    return navigableSet(navigableSet, null);

  }

  @GwtIncompatible // NavigableSet
  static <E extends @Nullable Object> NavigableSet<E> navigableSet(
      NavigableSet<E> navigableSet, @Nullable Object mutex) {
    return new SynchronizedNavigableSet<>(navigableSet, mutex);
  }

  @GwtIncompatible // NavigableSet
  static <E extends @Nullable Object> NavigableSet<E> navigableSet(NavigableSet<E> navigableSet) {
    return navigableSet(navigableSet, null);

    }
  }

  @GwtIncompatible // NavigableSet
  private static final class FilteredNavigableSet<E extends @Nullable Object>
      extends FilteredSortedSet<E> implements NavigableSet<E> {
    FilteredNavigableSet(NavigableSet<E> unfiltered, Predicate<? super E> predicate) {
      super(unfiltered, predicate);
    }

    NavigableSet<E> unfiltered() {
      return (NavigableSet<E>) unfiltered;
    }

    }
  }

  @GwtIncompatible // NavigableSet
  private static final class FilteredNavigableSet<E extends @Nullable Object>
      extends FilteredSortedSet<E> implements NavigableSet<E> {
    FilteredNavigableSet(NavigableSet<E> unfiltered, Predicate<? super E> predicate) {
      super(unfiltered, predicate);
    }

    NavigableSet<E> unfiltered() {
      return (NavigableSet<E>) unfiltered;
    }

    }

    @Override
    <E extends @Nullable Object> Collection<E> unmodifiableCollectionSubclass(
        Collection<E> collection) {
      if (collection instanceof NavigableSet) {
        return Sets.unmodifiableNavigableSet((NavigableSet<E>) collection);
      } else if (collection instanceof SortedSet) {
        return Collections.unmodifiableSortedSet((SortedSet<E>) collection);
      } else if (collection instanceof Set) {

    }

    @Override
    <E extends @Nullable Object> Collection<E> unmodifiableCollectionSubclass(
        Collection<E> collection) {
      if (collection instanceof NavigableSet) {
        return Sets.unmodifiableNavigableSet((NavigableSet<E>) collection);
      } else if (collection instanceof SortedSet) {
        return Collections.unmodifiableSortedSet((SortedSet<E>) collection);
      } else if (collection instanceof Set) {

import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.NavigableMap;
import java.util.NavigableSet;
import java.util.Properties;
import java.util.Set;
import java.util.SortedMap;
import java.util.SortedSet;
import java.util.TreeMap;
import java.util.concurrent.ConcurrentMap;
import junit.framework.TestCase;