code: def add_assertion(self, assertion, agent, agent_label, date=None):
          
          self.model.addIndividualToGraph(assertion, None, self.globaltt['assertion'])
  
          self.add_agent_to_graph(agent, agent_label, self.globaltt['organization'])
  
          self.graph.addTriple(
              assertion, self.globaltt['created_by'], agent)
  
          if date is not None:
              self.graph.addTriple(
                  self.graph, assertion, self.globaltt['date_created'], date)
  
          return
docs: Add assertion to graph
          :param assertion:
          :param agent:
          :param evidence_line:
          :param date:
          :return: None