code: func AttachStack(err error) {
  	if err == nil {
  		return
  	}
  	cast, ok := err.(*Error)
  	if !ok {
  		return
  	}
  	if len(cast.stacks) < 1 {
  		// only record stacks if this error was supposed to
  		return
  	}
  	cast.stacks = append(cast.stacks, getStack(2))
  }
docs: // AttachStack adds another stack to the current error's stack trace if it
  // exists