docs: def insert_draft_child(self, child_pid):
          """Insert a draft child to versioning."""
          if child_pid.status != PIDStatus.RESERVED:
              raise PIDRelationConsistencyError(
                  "Draft child should have status 'RESERVED'")
  
          if not self.draft_child:
              with db.session.begin_nested():
                  super(PIDNodeVersioning, self).insert_child(child_pid,
                                                              index=-1)
          else:
              raise PIDRelationConsistencyError(
                  "Draft child already exists for this relation: {0}".format(
                      self.draft_child))