code: def get_reviewer(self, r_term):
          
          reviewer_list = list(self.graph.triples((r_term, self.spdx_namespace['reviewer'], None)))
          if len(reviewer_list) != 1:
              self.error = True
              msg = 'Review must have exactly one reviewer'
              self.logger.log(msg)
              return
          try:
              return self.builder.create_entity(self.doc, six.text_type(reviewer_list[0][2]))
          except SPDXValueError:
              self.value_error('REVIEWER_VALUE', reviewer_list[0][2])
docs: Returns reviewer as creator object or None if failed.
          Reports errors on failure.