fieldStart := 0
	i := 0
	// Skip spaces in the front of the input.
	for i < len(s) && asciiSpace[s[i]] != 0 {
		i++
	}
	fieldStart = i
	for i < len(s) {
		if asciiSpace[s[i]] == 0 {
			i++
			continue
		}
		a[na] = s[fieldStart:i:i]
		na++
		i++
		// Skip spaces in between fields.
		for i < len(s) && asciiSpace[s[i]] != 0 {
			i++
		}
		fieldStart = i
	}

// but without constructing the slice.
func FieldsSeq(s []byte) iter.Seq[[]byte] {
	return func(yield func([]byte) bool) {
		start := -1
		for i := 0; i < len(s); {
			size := 1
			r := rune(s[i])
			isSpace := asciiSpace[s[i]] != 0
			if r >= utf8.RuneSelf {
				r, size = utf8.DecodeRune(s[i:])
				isSpace = unicode.IsSpace(r)
			}
			if isSpace {
				if start >= 0 {
					if !yield(s[start:i:i]) {
						return
					}