code: def add_external_reference_to_term(self,term_id, external_ref):
          
          if self.term_layer is not None:
              self.term_layer.add_external_reference(term_id, external_ref)
docs: Adds an external reference to the given term identifier
          @type term_id: string
          @param term_id: the term identifier
          @param external_ref: an external reference object
          @type external_ref: L{CexternalReference}