code: def get_annotation_comment(self, r_term):
          
          comment_list = list(self.graph.triples((r_term, RDFS.comment, None)))
          if len(comment_list) > 1:
              self.error = True
              msg = 'Annotation can have at most one comment.'
              self.logger.log(msg)
              return
          else:
              return six.text_type(comment_list[0][2])
docs: Returns annotation comment or None if found none or more than one.
          Reports errors.