docs: def add_rel(self, source_id, target_id, rel):
          """ Add a relationship: source, target must already exist (see add_node)
              'rel' is the name of the relationship 'contains' or whatever.
              Args:
                  source_id: the unique node_id of the source
                  target_id: the unique node_id of the target
                  rel: name of the relationship
              Returns:
                  Nothing
          """
          self.neo_db.add_rel(source_id, target_id, rel)