// bytes of sep to include in the results (none or all).
func splitSeq(s, sep []byte, sepSave int) iter.Seq[[]byte] {
	return func(yield func([]byte) bool) {
		if len(sep) == 0 {
			for len(s) > 0 {
				_, size := utf8.DecodeRune(s)
				if !yield(s[:size:size]) {
					return
				}
				s = s[size:]
			}
			return
		}
		for {
			i := Index(s, sep)
			if i < 0 {
				break
			}
			frag := s[:i+sepSave]

	return len(s) == len(t)

hasUnicode:
	s = s[i:]
	t = t[i:]
	for len(s) != 0 && len(t) != 0 {
		// Extract first rune from each.
		sr, size := utf8.DecodeRune(s)
		s = s[size:]
		tr, size := utf8.DecodeRune(t)
		t = t[size:]

		// If they match, keep going; if not, return false.

		// Easy case.
		if tr == sr {
			continue
		}

	start := 0
	for width := 0; start < len(data); start += width {
		var r rune
		r, width = utf8.DecodeRune(data[start:])
		if !isSpace(r) {
			break
		}
	}
	// Scan until space, marking end of word.
	for width, i := 0, start; i < len(data); i += width {
		var r rune
		r, width = utf8.DecodeRune(data[i:])
		if isSpace(r) {
			return i + width, data[start:i], nil
		}
	}

	if r.i >= int64(len(r.s)) {
		r.prevRune = -1
		return 0, 0, io.EOF
	}
	r.prevRune = int(r.i)
	if c := r.s[r.i]; c < utf8.RuneSelf {
		r.i++
		return rune(c), 1, nil
	}
	ch, size = utf8.DecodeRune(r.s[r.i:])
	r.i += int64(size)
	return
}

// UnreadRune complements [Reader.ReadRune] in implementing the [io.RuneScanner] interface.
func (r *Reader) UnreadRune() error {
	if r.i <= 0 {

		var i, runeCount int
		var expect rune
		// Use a string range loop to validate the sequence of runes.
		for i, expect = range test {
			if !s.Scan() {
				break
			}
			runeCount++
			got, _ := utf8.DecodeRune(s.Bytes())
			if got != expect {
				t.Errorf("#%d: %d: expected %q got %q", n, i, expect, got)
			}
		}
		if s.Scan() {
			t.Errorf("#%d: scan ran too long, got %q", n, s.Text())
		}

		b.Reset()
		return 0, 0, io.EOF
	}
	c := b.buf[b.off]
	if c < utf8.RuneSelf {
		b.off++
		b.lastRead = opReadRune1
		return rune(c), 1, nil
	}
	r, n := utf8.DecodeRune(b.buf[b.off:])
	b.off += n
	b.lastRead = readOp(n)
	return r, n, nil
}

// UnreadRune unreads the last rune returned by [Buffer.ReadRune].
// If the most recent read or write operation on the buffer was

		b.fill() // b.w-b.r < len(buf) => buffer is not full
	}
	b.lastRuneSize = -1
	if b.r == b.w {
		return 0, 0, b.readErr()
	}
	r, size = utf8.DecodeRune(b.buf[b.r:b.w])
	b.r += size
	b.lastByte = int(b.buf[b.r-1])
	b.lastRuneSize = size
	return r, size, nil
}

// UnreadRune unreads the last rune. If the most recent method called on

					i += len(old)
					n--
					if len(old) != 0 {
						continue
					}
					if i == len(in) {
						break
					}
				}
				if len(old) == 0 {
					_, length := utf8.DecodeRune(in[i:])
					out.Write(in[i : i+length])
					i += length
				} else {
					out.WriteByte(in[i])
					i++
				}
			}
			if len(old) == 0 && n != 0 {
				out.Write(new)
			}

pkg unicode, var Z *RangeTable
pkg unicode, var Zl *RangeTable
pkg unicode, var Zp *RangeTable
pkg unicode, var Zs *RangeTable
pkg unicode/utf16, func Decode([]uint16) []int32
pkg unicode/utf16, func DecodeRune(int32, int32) int32
pkg unicode/utf16, func Encode([]int32) []uint16
pkg unicode/utf16, func EncodeRune(int32) (int32, int32)
pkg unicode/utf16, func IsSurrogate(int32) bool