sS ai_makeNGramScorer(Cl<S> pos, Cl<S> neg) {
  Set<S> common = setIntersection(asSet(pos), neg);
  pos = listMinusSet(pos, common);
  neg = listMinusSet(neg, common);
  S s = join("|", pos);
  if (nempty(neg)) s += "^" + join("|", neg);
  ret s;
}