code: def add_external_reference_to_entity(self,entity_id,ext_ref):
          
          node_entity = self.map_entity_id_to_node.get(entity_id)
          if node_entity is not None:
              entity = Centity(node_entity,self.type)
              entity.add_external_reference(ext_ref)
          else:
              print>>sys.stderr,'Trying to add a reference to the entity',entity_id,'but can not be found in this file'
docs: Adds an external reference to a entity specified by the entity identifier
          @param entity_id: the entity identifier
          @type entity_id: string
          @param ext_ref: the external reference
          @type ext_ref: L{CexternalReference}