code: func AuthenticateExecution(c context.Context, auth *dm.Execution_Auth) (a *Attempt, e *Execution, err error) {
  	a, e, err = verifyExecutionAndCheckExTok(c, auth)
  	if err != nil {
  		logging.Fields{ek: err, "eid": auth.Id}.Errorf(c, "failed to verify execution")
  		err = makeError(err, "requires execution Auth")
  	}
  	return a, e, err
  }
docs: // AuthenticateExecution verifies that the Attempt is executing, and that evkey
  // matches the execution key of the current Execution for this Attempt.
  //
  // As a bonus, it will return the loaded Attempt and Execution.