sS nlLogic_render(IfThen rule) {
  if (rule == null) null;
  ret nlLogic_render(rule.in) + " => " + nlLogic_render(rule.out);
}

sS nlLogic_render(Exp e) {
  if (e cast Func) ret e.name + "(" + nlLogic_render(e.arg) + ")";
  if (e cast Sentence) ret join(e.tok);
  if (e cast And) ret nlLogic_render(e.a) + " && " + nlLogic_render(e.b);
  ret "TODO: " + className(e);
}