code: func WithBackoff(bf BackoffFunc) CallOption {
  	return CallOption{applyFunc: func(o *options) {
  		o.backoffFunc = BackoffFuncContext(func(ctx context.Context, attempt uint) time.Duration {
  			return bf(attempt)
  		})
  	}}
  }
docs: // WithBackoff sets the `BackoffFunc` used to control time between retries.