docs: def _create_chain(pid, sid):
      """Create the initial chain structure for a new standalone object. Intended to be
      called in MNStorage.create().
  
      Preconditions:
      - ``sid`` must be verified to be available to be assigned to a new standalone
        object. E.g., with is_valid_sid_for_new_standalone().
  
      """
      chain_model = d1_gmn.app.models.Chain(
          # sid=d1_gmn.app.models.did(sid) if sid else None,
          head_pid=d1_gmn.app.did.get_or_create_did(pid)
      )
      chain_model.save()
      _add_pid_to_chain(chain_model, pid)
      _set_chain_sid(chain_model, sid)
      return chain_model