static L<Pair<S>> ai_texts_dollarXY(S a, S b, S c) {
  if (isDollarVar(b)) todo();
  new L<Pair<S>> l;
  if (eq(a, "$X") && eq(c, "$Y")) {
    for (WebRelation r : indexedRelations(b))
      l.add(pair(r.a, r.b));
    ret l;
  }
  if (eq(a, "$Y") && eq(c, "$X")) {
    for (WebRelation r : indexedRelations(b))
      l.add(pair(r.b, r.a));
    ret l;
  }
  fail("improper arguments: " + a + "/" + b + "/" + c);
}