svoid nlLogic_visit(IfThen rule, VF1<O> visitor) {
  if (rule == null) ret;
  callF(visitor, rule);
  nlLogic_visit(rule.in, visitor);
  nlLogic_visit(rule.out, visitor);
}

svoid nlLogic_visit(Exp e, VF1<O> visitor) {
  if (e == null) ret;
  callF(visitor, e);
  if (e cast And) {
    nlLogic_visit(e.a);
    nlLogic_visit(e.b);
  } else if (e cast ExpNot)
    nlLogic_visit(e.a);
}