def add_external_reference_to_entity(self,entity_id, external_ref):
        """
        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}
        """
        if self.entity_layer is not None:
            self.entity_layer.add_external_reference_to_entity(entity_id,external_ref)
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}