code: func RootCause(e error) error {
  	if e == nil {
  		return e
  	}
  	for {
  		c := Cause(e)
  		if c == nil {
  			break
  		} else {
  			e = c
  		}
  	}
  	return e
  }
docs: // RootCause returns the innermost cause of the argument (i.e. the last
  // error in the cause chain)