code: func (rw *RetryWaiter) WaitIf(failure bool) <-chan struct{} {
  	if failure {
  		return rw.Failed()
  	}
  	return rw.Success()
  }
docs: // WaitIf is a convenice method to record whether the last
  // operation was a success or failure and return a chan that
  // will be selectablw when the next operation can be done.