docs: def remove_child(self, child_pid):
          """Remove a child from a PID concept."""
          with db.session.begin_nested():
              if not isinstance(child_pid, PersistentIdentifier):
                  child_pid = resolve_pid(child_pid)
              relation = PIDRelation.query.filter_by(
                  parent=self._resolved_pid,
                  child=child_pid,
                  relation_type=self.relation_type.id).one()
              db.session.delete(relation)