func (b *BufferedSeriesIterator) PeekBack(n int) (t int64, v float64, ok bool) {
	return b.buf.nthLast(n)
}
func (b *BufferedSeriesIterator) PeekBack(n int) (t int64, v float64, ok bool) {
	return b.buf.nthLast(n)
}
// PeekBack returns the nth previous element of the iterator. If there is none buffered,
// ok is false.