code: func (s *memSeries) truncateChunksBefore(mint int64) (removed int) {
  	var k int
  	for i, c := range s.chunks {
  		if c.maxTime >= mint {
  			break
  		}
  		k = i + 1
  	}
  	s.chunks = append(s.chunks[:0], s.chunks[k:]...)
  	s.firstChunkID += k
  	if len(s.chunks) == 0 {
  		s.headChunk = nil
  	} else {
  		s.headChunk = s.chunks[len(s.chunks)-1]
  	}
  
  	return k
  }
docs: // truncateChunksBefore removes all chunks from the series that have not timestamp
  // at or after mint. Chunk IDs remain unchanged.