type Element2[TElem any] struct {
	next, prev *Element2[TElem]
	list *List2[TElem]
	Value TElem
}

type List2[TElem any] struct {
	root Element2[TElem]
	len  int
}

func (l *List2[TElem]) Init() *List2[TElem] {
	l.root.next = &l.root
	l.root.prev = &l.root
	l.len = 0
	return l
}

// Self-recursive instantiations must work correctly.
type A[P any] struct { _ *A[P] }

	//	q/s/u
	//	q/v
	//	w
	//
	// is actually sorted as:
	//
	//	p       # dir=.    elem=p
	//	q/      # dir=.    elem=q
	//	w/      # dir=.    elem=w
	//	q/r     # dir=q    elem=r
	//	q/s/    # dir=q    elem=s
	//	q/v     # dir=q    elem=v
	//	q/s/t   # dir=q/s  elem=t
	//	q/s/u   # dir=q/s  elem=u
	//
	// This order brings directory contents together in contiguous sections

		for fname, f := range fsys {
			if strings.HasPrefix(fname, prefix) {
				felem := fname[len(prefix):]
				i := strings.Index(felem, "/")
				if i < 0 {
					list = append(list, mapFileInfo{felem, f})
				} else {
					need[fname[len(prefix):len(prefix)+i]] = true
				}
			}
		}
		// If the directory name is not in the map,
		// and there are no children of the name in the map,
		// then the directory is treated as not existing.

		// more detail.
		print("runtime: nelems=", s.nelems, " nalloc=", nalloc, " previous allocCount=", s.allocCount, " nfreed=", nfreed, "\n")
		throw("sweep increased allocation count")
	}

	s.allocCount = nalloc
	s.freeindex = 0 // reset allocation index to start of span.
	s.freeIndexForScan = 0
	if traceEnabled() {
		getg().m.p.ptr().trace.reclaimed += uintptr(nfreed) * s.elemsize
	}

func Make[Elem comparable]() Set[Elem] {
	return Set[Elem]{m: make(map[Elem]struct{})}
}

// Add adds an element to a set.
func (s Set[Elem]) Add(v Elem) {
	s.m[v] = struct{}{}
}

// Delete removes an element from a set. If the element is not present
// in the set, this does nothing.
func (s Set[Elem]) Delete(v Elem) {
	delete(s.m, v)
}

// Contains reports whether v is in the set.

package set

type Set[Elem comparable] map[Elem]struct{}

func Make[Elem comparable]() Set[Elem] {
	return make(Set(Elem))
}

func (s Set[Elem]) Add(v Elem) {
	s[v] = struct{}{}
}

func (s Set[Elem]) Delete(v Elem) {
	delete(s, v)
}

func (s Set[Elem]) Contains(v Elem) bool {
	_, ok := s[v]
	return ok
}

func (s Set[Elem]) Len() int {
	return len(s)
}

// until c is closed or the context is canceled, at which point the
// returned channel is closed.
func Filter[Elem any](ctx context.Context, c <-chan Elem, f func(Elem) bool) <-chan Elem {
	r := make(chan Elem)
	go func(ctx context.Context, c <-chan Elem, f func(Elem) bool, r chan<- Elem) {
		defer close(r)
		for {
			select {
			case <-ctx.Done():
				return
			case v, ok := <-c:
				if !ok {
					return

		~float32 | ~float64 |
		~string
}

type orderedSlice[Elem Ordered] []Elem

func (s orderedSlice[Elem]) Len() int { return len(s) }
func (s orderedSlice[Elem]) Less(i, j int) bool {
	if s[i] < s[j] {
		return true
	}
	isNaN := func(f Elem) bool { return f != f }
	if isNaN(s[i]) && !isNaN(s[j]) {
		return true
	}
	return false
}
func (s orderedSlice[Elem]) Swap(i, j int) { s[i], s[j] = s[j], s[i] }

// until c is closed or the context is canceled, at which point the
// returned channel is closed.
func _Filter[Elem any](ctx context.Context, c <-chan Elem, f func(Elem) bool) <-chan Elem {
	r := make(chan Elem)
	go func(ctx context.Context, c <-chan Elem, f func(Elem) bool, r chan<- Elem) {
		defer close(r)
		for {
			select {
			case <-ctx.Done():
				return
			case v, ok := <-c:
				if !ok {
					return

type sliceFn[Elem any] struct {
	s []Elem
	f func(Elem, Elem) bool
}

func (s sliceFn[Elem]) Len() int           { return len(s.s) }
func (s sliceFn[Elem]) Less(i, j int) bool { return s.f(s.s[i], s.s[j]) }
func (s sliceFn[Elem]) Swap(i, j int)      { s.s[i], s.s[j] = s.s[j], s.s[i] }

// SliceFn sorts the slice s according to the function f.
func SliceFn[Elem any](s []Elem, f func(Elem, Elem) bool) {
	Sort(sliceFn[Elem]{s, f})