static L<S> ai_tok_combinePhrases(L<S> tok) {
  new L<S> l;
  int n = l(tok);
  for i over tok: {
    S t = tok.get(i);
    if (odd(i) && i+2 < n) {
      S phrase = t + " " + tok.get(i+2);
      if (isKnownPhrase(phrase)) {
        l.add(joinSubList(tok, i, i+3));
        i += 2;
        continue;
      }
      l.add(t);
    }
    l.add(t);
  }
  ret l;
}