code: public void write(ByteAppendable os, char ch)
      throws IOException
    {
      if (ch < 0x80)
        os.write(ch);
      else {
        os.write('\\');
        os.write('u');
  
        int b = (ch >> 12) & 0xf;
        os.write(b < 10 ? b + '0' : b + 'a' - 10);
        b = (ch >> 8) & 0xf;
        os.write(b < 10 ? b + '0' : b + 'a' - 10);
        b = (ch >> 4) & 0xf;
        os.write(b < 10 ? b + '0' : b + 'a' - 10);
        b = ch & 0xf;
        os.write(b < 10 ? b + '0' : b + 'a' - 10);
      }
    }
docs: Writes a character to the output stream with the correct encoding.
  
  @param ch the character to write.