code: func (op *op) apply(b *Bitmap) (changed bool) {
  	switch op.typ {
  	case opTypeAdd:
  		return b.DirectAdd(op.value)
  	case opTypeRemove:
  		return b.remove(op.value)
  	case opTypeAddBatch:
  		changed = b.DirectAddN(op.values...) > 0
  	case opTypeRemoveBatch:
  		changed = b.DirectRemoveN(op.values...) > 0
  	default:
  		panic(fmt.Sprintf("invalid op type: %d", op.typ))
  	}
  	return changed
  }
docs: // apply executes the operation against a bitmap.