code: def add_external_reference_to_role(self,role_id,external_ref):
          
          if self.srl_layer is not None:
              self.srl_layer.add_external_reference_to_role(role_id,external_ref)
docs: Adds an external reference to the given role identifier in the SRL layer
          @type role_id: string
          @param role_id: the role identifier
          @param external_ref: an external reference object
          @type external_ref: L{CexternalReference}