code: func unionBitmapRunInPlace(a, b *Container) {
  	a.unmapBitmap()
  	bitmap := a.bitmap()
  	statsHit("union/BitmapRun")
  	for _, run := range b.runs() {
  		bitmapSetRangeIgnoreN(bitmap, uint64(run.start), uint64(run.last)+1)
  	}
  }
docs: // unions the run b into the bitmap a, mutating a in place. The n value of
  // a will need to be repaired after the fact.