code: def label(self, id):
          
          if self.ontology is not None:
              label = self.ontology.label(id)
              if label is not None:
                  return label
          if self.subject_label_map is not None and id in self.subject_label_map:
              return self.subject_label_map[id]
          return None
docs: return label for a subject id
  
          Will make use of both the ontology and the association set