code: func (r *reader) ReadRune() (ch rune, size int, err error) {
  	ch, _ = r.read()
  	if ch == eof {
  		err = io.EOF
  	}
  	return
  }
docs: // ReadRune reads the next rune from the reader.
  // This is a wrapper function to implement the io.RuneReader interface.
  // Note that this function does not return size.