code: func SingleError(err error) error {
  	if me, ok := err.(MultiError); ok {
  		if len(me) == 0 {
  			return nil
  		}
  		return me[0]
  	}
  	return err
  }
docs: // SingleError provides a simple way to uwrap a MultiError if you know that it
  // could only ever contain one element.
  //
  // If err is a MultiError, return its first element. Otherwise, return err.