code: public List<OPFItem> getItems()
    {
      return (items != null) ? items.getItems() : ImmutableList.<OPFItem> of();
    }
docs: Returns an immutable list of all the items in the OPF, in document order,
  including those represented by <code>link</code> elements pointing to local
  resources.
  
  <p>
  Returns the empty list if the items have not been parsed yet.
  </p>
  
  @return the list of all items, guaranteed non-null.