static LS tok_joinExtendedUnicodes(LS tok) {
  new LS tok2;
  for i over tok: {
    S t = tok.get(i);
    if (odd(i) && isHighSurrogate(t)
      && empty(tok.get(i+1))
      && isLowSurrogate(get(tok, i+2))) {
      tok2.add(tok.get(i) + tok.get(i+2));
      i += 2;
    } else
      tok2.add(t);
  }
  ret tok2;
}