static void ai_spec_groupingPrefix(S s) {
  ai_spec_groupingPrefix(s, ai_groupingPrefixes());
}

static void ai_spec_groupingPrefix(S s, Set<S> prefixes) {
  new Matches m;
  for (S prefix : prefixes)
    if (swic(s, prefix + " ", m))
      ai_postTriple(s, "groups to", prefix + " " + curlyBraces(m.rest()));
}