code: func differenceBitmapRun(a, b *Container) *Container {
  	statsHit("difference/BitmapRun")
  	if a.n == 0 || b.n == 0 {
  		return a.Clone()
  	}
  
  	output := a.Clone()
  	for _, run := range b.runs() {
  		output.bitmapZeroRange(uint64(run.start), uint64(run.last)+1)
  	}
  	return output
  }
docs: // differenceBitmapRun computes the difference of an bitmap from a run.