docs: def relate_to(self, target, reltype, is_external=False):
          """
          Return rId key of relationship of *reltype* to *target*, from an
          existing relationship if there is one, otherwise a newly created one.
          """
          if is_external:
              return self.rels.get_or_add_ext_rel(reltype, target)
          else:
              rel = self.rels.get_or_add(reltype, target)
              return rel.rId