}

  @Override
  public @Nullable Entry<K, V> pollFirstEntry() {
    return delegate.pollFirstEntry();
  }

  @Override
  public @Nullable Entry<K, V> pollLastEntry() {
    return delegate.pollLastEntry();
  }

  @Override
  public @Nullable V put(K key, V value) {
    return delegate.put(checkValid(key), value);
  }

  @Override

    return delegate().lastEntry();
  }

  @Override
  public @Nullable Entry<E> pollFirstEntry() {
    throw new UnsupportedOperationException();
  }

  @Override
  public @Nullable Entry<E> pollLastEntry() {
    throw new UnsupportedOperationException();
  }

  @Override
  public SortedMultiset<E> headMultiset(E upperBound, BoundType boundType) {

      result = Multisets.immutableEntry(result.getElement(), result.getCount());
      entryIterator.remove();
      return result;
    }
    return null;
  }

  @Override
  public @Nullable Entry<E> pollLastEntry() {
    Iterator<Entry<E>> entryIterator = descendingEntryIterator();
    if (entryIterator.hasNext()) {
      Entry<E> result = entryIterator.next();

    return delegate().lastEntry();
  }

  @Override
  public @Nullable Entry<E> pollFirstEntry() {
    throw new UnsupportedOperationException();
  }

  @Override
  public @Nullable Entry<E> pollLastEntry() {
    throw new UnsupportedOperationException();
  }

  @Override
  public SortedMultiset<E> headMultiset(@ParametricNullness E upperBound, BoundType boundType) {

  }

  @Override
  public @Nullable Entry<K, V> pollFirstEntry() {
    return Iterators.pollNext(entryIterator());
  }

  @Override
  public @Nullable Entry<K, V> pollLastEntry() {
    return Iterators.pollNext(descendingEntryIterator());
  }

  @Override
  @ParametricNullness
  public K firstKey() {
    Entry<K, V> entry = firstEntry();
    if (entry == null) {

      return standardLastEntry();
    }

    @Override
    public Entry<E> pollFirstEntry() {
      return standardPollFirstEntry();
    }

    @Override
    public Entry<E> pollLastEntry() {
      return standardPollLastEntry();
    }

    @Override
    public SortedMultiset<E> subMultiset(
        E lowerBound, BoundType lowerBoundType, E upperBound, BoundType upperBoundType) {

  }

  @Override
  public @Nullable Entry<K, V> pollFirstEntry() {
    return Iterators.pollNext(entryIterator());
  }

  @Override
  public @Nullable Entry<K, V> pollLastEntry() {
    return Iterators.pollNext(descendingEntryIterator());
  }

  @Override
  @ParametricNullness
  public K firstKey() {
    Entry<K, V> entry = firstEntry();
    if (entry == null) {

    public @Nullable E pollFirst() {
      return getElementOrNull(multiset().pollFirstEntry());
    }

    @Override
    public @Nullable E pollLast() {
      return getElementOrNull(multiset().pollLastEntry());
    }

    @Override
    public NavigableSet<E> subSet(
        @ParametricNullness E fromElement,
        boolean fromInclusive,
        @ParametricNullness E toElement,

      return standardLastEntry();
    }

    @Override
    public Entry<E> pollFirstEntry() {
      return standardPollFirstEntry();
    }

    @Override
    public Entry<E> pollLastEntry() {
      return standardPollLastEntry();
    }

    @Override
    public SortedMultiset<E> subMultiset(
        E lowerBound, BoundType lowerBoundType, E upperBound, BoundType upperBoundType) {

   */
  protected @Nullable Entry<K, V> standardPollFirstEntry() {
    return Iterators.pollNext(entrySet().iterator());
  }

  @Override
  public @Nullable Entry<K, V> pollLastEntry() {
    return delegate().pollLastEntry();
  }

  /**
   * A sensible definition of {@link #pollFirstEntry} in terms of the {@code iterator} of the {@code