sbool ai_deriveFacts_debug;

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