func (r *sampleRing) nthLast(n int) (int64, float64, bool) {
	if n > r.l {
		return 0, 0, false
	}
	t, v := r.at(r.l - n)
	return t, v, true
}
func (r *sampleRing) nthLast(n int) (int64, float64, bool) {
	if n > r.l {
		return 0, 0, false
	}
	t, v := r.at(r.l - n)
	return t, v, true
}
// nthLast returns the nth most recent element added to the ring.