// 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
		}

		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