code: func (b *Bitmap) removeEmptyContainers() {
  	citer, _ := b.Containers.Iterator(0)
  	for citer.Next() {
  		k, c := citer.Value()
  		if c.n == 0 {
  			b.Containers.Remove(k)
  		}
  	}
  }
docs: // removeEmptyContainers deletes all containers that have a count of zero.