func withBackoff(bf backoffFunc) retryOption {
	return retryOption{applyFunc: func(o *options) {
		o.backoffFunc = bf
	}}
}
func withBackoff(bf backoffFunc) retryOption {
	return retryOption{applyFunc: func(o *options) {
		o.backoffFunc = bf
	}}
}
// WithBackoff sets the `BackoffFunc `used to control time between retries.