code: @Override
      public SortedSet<Cell<V>> cells() {
          return new ForwardingSortedSet<Cell<V>>() {
              @Override
              protected SortedSet<Cell<V>> delegate() {
                  return cells;
              }
              @Override
              public boolean add(Cell<V> element) {
                  return super.add(ImmutableCell.copyOf(element));
              }
              @Override
              public boolean addAll(Collection<? extends Cell<V>> collection) {
                  return super.standardAddAll(collection);
              }
          };
      }
docs: -----------------------------------------------------------------------