static S ai_mechList_placeholdersToRandomEntry(S s) {
  L<S> tok = javaTokWithAngleBrackets(s);
  for (int i = 1; i < l(tok); i += 2) {
    S t = tok.get(i);
    if (isAngleBracketed(t)) {
      t = deAngleBracket(t);
      S list = or(ai_placeHolderToList(t), t);
      L<S> entries = mechList(list);
      if (entries == null)
        fail("No list for <" + t + ">");
      tok.set(i, random(entries));
    }
  }
  ret join(tok);
}