code: func (pc *PyramidChunker) cleanChunkLevels() {
  	for i, l := range pc.chunkLevel {
  		if l == nil {
  			pc.chunkLevel = append(pc.chunkLevel[:i], append(pc.chunkLevel[i+1:], nil)...)
  		}
  	}
  }
docs: // cleanChunkLevels removes gaps (nil levels) between chunk levels
  // that are not nil.