code: public Set<URI> getNonTopicrefReferenceSet() {
          final Set<URI> res = new HashSet<>(nonTopicrefReferenceSet);
          res.removeAll(normalProcessingRoleSet);
          res.removeAll(resourceOnlySet);
          return res;
      }
docs: List of files referenced by something other than topicref
  
  @return the resource-only set