code: func (ch Challenge) CheckConsistencyForClientOffer() error {
  	if err := ch.checkConsistency(); err != nil {
  		return err
  	}
  
  	// Before completion, the key authorization field should be empty
  	if ch.ProvidedKeyAuthorization != "" {
  		return fmt.Errorf("A response to this challenge was already submitted.")
  	}
  	return nil
  }
docs: // CheckConsistencyForClientOffer checks the fields of a challenge object before it is
  // given to the client.