func Or(first BitMask, masks ...BitMask) BitMask {
	s := *first.(*bitMask)
	s.Or(masks...)
	return &s
}

// IterateBitMasks iterates all possible masks from a list of bits,
// issuing a callback on each mask.
func IterateBitMasks(bits []int, callback func(BitMask)) {
	var iterate func(bits, accum []int, size int)
	iterate = func(bits, accum []int, size int) {
		if len(accum) == size {