static S escapeMultichars(S s) {
  if (!hasExtendedUnicodeCharacters(s)) ret s;
  new StringBuilder buf;
  for (int i = 0; i < l(s); i++)
    if (isExtendedUnicodeCharacter(s, i)) {
      buf.append("[xchar " + intToHex(s.codePointAt(i)) + "]");
      ++i;
    } else
      buf.append(s.charAt(i));
  ret str(buf);
}