}

    @Override
    public @Nullable E pollFirst() {
      return Iterables.removeFirstMatching(unfiltered(), predicate);
    }

    @Override
    public @Nullable E pollLast() {
      return Iterables.removeFirstMatching(unfiltered().descendingSet(), predicate);
    }

    @Override
    public NavigableSet<E> descendingSet() {

      list.remove(n);
    }
  }

  /** Removes and returns the first matching element, or returns {@code null} if there is none. */
  static <T extends @Nullable Object> @Nullable T removeFirstMatching(
      Iterable<T> removeFrom, Predicate<? super T> predicate) {
    checkNotNull(predicate);
    Iterator<T> iterator = removeFrom.iterator();
    while (iterator.hasNext()) {
      T next = iterator.next();

    return Iterators.removeIf(removeFrom.iterator(), predicate);
  }

  /** Removes and returns the first matching element, or returns {@code null} if there is none. */
  static <T extends @Nullable Object> @Nullable T removeFirstMatching(
      Iterable<T> removeFrom, Predicate<? super T> predicate) {
    checkNotNull(predicate);
    Iterator<T> iterator = removeFrom.iterator();
    while (iterator.hasNext()) {
      T next = iterator.next();