code: def add_external_reference_to_entity(self,entity_id, external_ref):
          
          if self.entity_layer is not None:
              self.entity_layer.add_external_reference_to_entity(entity_id,external_ref)
docs: Adds an external reference to the given entity identifier in the entity layer
          @type entity_id: string
          @param entity_id: the entity identifier
          @param external_ref: an external reference object
          @type external_ref: L{CexternalReference}