docs: def add_external_reference(self,term_id, external_ref):
          """
          Adds an external reference for the given term
          @type term_id: string
          @param term_id: the term identifier
          @type external_ref: L{CexternalReference}
          @param external_ref: the external reference object
          """
          if term_id in self.idx:
              term_obj = Cterm(self.idx[term_id],self.type)
              term_obj.add_external_reference(external_ref)
          else:
              print('{term_id} not in self.idx'.format(**locals()))