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