docs: def _update_sid_to_last_existing_pid_map(pid):
      """Set chain head PID to the last existing object in the chain to which ``pid``
      belongs. If SID has been set for chain, it resolves to chain head PID.
  
      Intended to be called in MNStorage.delete() and other chain manipulation.
  
      Preconditions:
      - ``pid`` must exist and be verified to be a PID.
        d1_gmn.app.views.asserts.is_existing_object()
  
      """
      last_pid = _find_head_or_latest_connected(pid)
      chain_model = _get_chain_by_pid(last_pid)
      if not chain_model:
          return
      chain_model.head_pid = d1_gmn.app.did.get_or_create_did(last_pid)
      chain_model.save()