func writeRune(w io.Writer, r rune) (int, error) {
if w, ok := w.(runeWriter); ok {
return w.WriteRune(r)
}
var encoded [4]byte
subset := encoded[:utf8.EncodeRune(encoded[:], r)]
if len(subset) == 0 {
return 0, nil
}
return w.Write(subset)
}