code: func IsNotFound(e error) bool {
  	type nf interface {
  		IsNotFoundError() bool
  	}
  	err := Unwrap(e)
  	_, ok := err.(nf)
  	if !ok {
  		return os.IsNotExist(err)
  	}
  	return ok
  }
docs: // IsNotFound returns whether this error is of NotFoundError type