code: public List<OPFItem> getSpineItems()
    {
      return (items != null) ? items.getSpineItems() : ImmutableList.<OPFItem> of();
    }
docs: Returns an immutable list of the items in the spine. May contain duplicates
  if several <code>itemref</code> elements point to the same item.
  <p>
  Returns the empty list if the items have not been parsed yet.
  </p>
  
  @return the list of items in the spine, guaranteed non-null.