code: func (e *Execution) Revoke(c context.Context) error {
  	e.Token = nil
  	if err := e.ModifyState(c, dm.Execution_STOPPING); err != nil {
  		return err
  	}
  	return ds.Put(c, e)
  }
docs: // Revoke will clear the Token and Put this Execution to the datastore. This
  // action requires the Execution to be in the RUNNING state, and causes it to
  // enter the STOPPING state.