static void ai_spec_groupingPrefix(S s) {
  L<S> tok = javaTok(s);
  for (int i = 1; i < l(tok)-2; i += 2) {
    S prefix = trimJoinSubList(tok, 0, i+1);
    if (ai_hasTriple(prefix, "is a", "grouping prefix"))
      ai_postTriple(s, "groups to",
        prefix + " " + curlyBraces(trimJoinSubList(tok, i+2)));
  }
}