code: func (pc *PyramidChunker) depth() (d int) {
  	for _, l := range pc.chunkLevel {
  		if l == nil {
  			return
  		}
  		d++
  	}
  	return
  }
docs: // depth returns the number of chunk levels.
  // It is used to detect if there is only one data chunk
  // left for the last branch.