code: func record(err error, depth int) error {
  	if err == nil {
  		return nil
  	}
  	cast, ok := err.(*Error)
  	if !ok {
  		return err
  	}
  	cast.exits = append(cast.exits, callerState(depth))
  	return cast
  }
docs: // record will record the pc at the given depth into the error if it is
  // capable of recording it.