Method filter

guava/src/com/google/common/collect/Sets.java:1187–1201

(
      Set<E> unfiltered, Predicate<? super E> predicate)

   */
  // TODO(kevinb): how to omit that last sentence when building GWT javadoc?
  public static <E extends @Nullable Object> Set<E> filter(
      Set<E> unfiltered, Predicate<? super E> predicate) {
    if (unfiltered instanceof SortedSet) {
      return filter((SortedSet<E>) unfiltered, predicate);
    }
    if (unfiltered instanceof FilteredSet) {
      // Support clear(), removeAll(), and retainAll() when filtering a filtered
      // collection.
      FilteredSet<E> filtered = (FilteredSet<E>) unfiltered;
      Predicate<E> combinedPredicate = Predicates.and(filtered.predicate, predicate);
      return new FilteredSet<>((Set<E>) filtered.unfiltered, combinedPredicate);
    }

    return new FilteredSet<>(checkNotNull(unfiltered), checkNotNull(predicate));
  }

  /**
   * Returns the elements of a {@code SortedSet}, {@code unfiltered}, that satisfy a predicate. The