r.i += int64(size)
	return
}

// UnreadRune complements [Reader.ReadRune] in implementing the [io.RuneScanner] interface.
func (r *Reader) UnreadRune() error {
	if r.i <= 0 {
		return errors.New("bytes.Reader.UnreadRune: at beginning of slice")
	}
	if r.prevRune < 0 {
		return errors.New("bytes.Reader.UnreadRune: previous operation was not ReadRune")
	}
	r.i = int64(r.prevRune)

	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
// not a successful [Buffer.ReadRune], UnreadRune returns an error.  (In this regard
// it is stricter than [Buffer.UnreadByte], which will unread the last byte
// from any read operation.)
func (b *Buffer) UnreadRune() error {
	if b.lastRead <= opInvalid {

		t.Error("unexpected error on ReadRune (1):", err)
	}
	if err = r.UnreadRune(); err != nil {
		t.Error("unexpected error on UnreadRune (1):", err)
	}
	if r.UnreadRune() == nil {
		t.Error("expected error after UnreadRune (1)")
	}
	// Test error after Read.
	_, _, err = r.ReadRune() // reset state
	if err != nil {
		t.Error("unexpected error on ReadRune (2):", err)
	}
	_, err = r.Read(buf)
	if err != nil {

		}
	}

	// Check that UnreadRune works
	buf.Reset()

	// check at EOF
	if err := buf.UnreadRune(); err == nil {
		t.Fatal("UnreadRune at EOF: got no error")
	}
	if _, _, err := buf.ReadRune(); err == nil {
		t.Fatal("ReadRune at EOF: got no error")
	}
	if err := buf.UnreadRune(); err == nil {
		t.Fatal("UnreadRune after ReadRune at EOF: got no error")
	}

	b.lastRuneSize = size
	return r, size, nil
}

// UnreadRune unreads the last rune. If the most recent method called on
// the [Reader] was not a [Reader.ReadRune], [Reader.UnreadRune] returns an error. (In this
// regard it is stricter than [Reader.UnreadByte], which will unread the last byte
// from any read operation.)
func (b *Reader) UnreadRune() error {
	if b.lastRuneSize < 0 || b.r < b.lastRuneSize {