// biasedSparseMap like a SparseSet.
type biasedSparseMap struct {
	s     *sparseMap
	first int
}

// newBiasedSparseMap returns a new biasedSparseMap for values between first and last, inclusive.
func newBiasedSparseMap(first, last int) *biasedSparseMap {
	if first > last {
		return &biasedSparseMap{first: math.MaxInt32, s: nil}
	}

func newXposmap(x map[int]lineRange) *xposmap {
	maps := make(map[int32]*biasedSparseMap)
	for i, p := range x {
		maps[int32(i)] = newBiasedSparseMap(int(p.first), int(p.last))
	}
	return &xposmap{maps: maps, lastIndex: -1} // zero for the rest is okay
}

// clear removes data from the map but leaves the sparse skeleton.
func (m *xposmap) clear() {