code: def link_reference(self, source, target):
          
          target_uid = api.get_uid(target)
          # get the annotation storage key
          key = self.get_relationship_key(target)
          # get all backreferences from the source
          # N.B. only like this we get the persistent mapping!
          backrefs = get_backreferences(source, relationship=None)
          if key not in backrefs:
              backrefs[key] = PersistentList()
          if target_uid not in backrefs[key]:
              backrefs[key].append(target_uid)
          return True
docs: Link the target to the source