static Set<S> ai_deriveFacts(L<IfThen> rules, LS facts) {
  final new NLLogicChecker_v3 c;
  final new NLLogicChecker_v2.Matching m;
  final new LinkedHashSet<S> outFacts;
  for (final IfThen rule : nlLogic_filterByRHSConstructs(rules, 'fact))
    c.iterate(rule.in, m, r {
      //print("Got matches: " + sfu(m.matches));
      Exp out = c.apply(rule.out, m);
      if (out cast Func)
        if (eq(out.name, "fact"))
          outFacts.add(nlLogic_text(out.arg));
    });
  ret outFacts;
}