code: 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
  }
docs: // nthLast returns the nth most recent element added to the ring.