code: public Optional<OPFItem> getResource(String path)
    {
      return (path == null || !resources.containsKey(path)) ? Optional.<OPFItem> absent()
          : Optional.of(resources.get(path).item);
    }
docs: Returns an {@link Optional} containing the Package Document item for the
  given Publication Resource path, or {@link Optional#absent()} if no resource
  has been registered for the given path.