static Set<Lisp> followBackwardRelation(S relation, S argument) {
  ret followBackwardRelation(relation, lisp(argument));
}

static Set<Lisp> followBackwardRelation(S relation, Lisp argument) {
  new HashSet<Lisp> out;
  for (Lisp l : lispTruthByHead(relation))
    if (eq(l.get(1), argument))
      out.add(l.get(0));
  ret out;
}