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