class A {
  public:
  A() { forward = 0; }
  attach(B* b) { forward = b; b->attach(this); }
  bool invariant() {
     return !forward || forward->backward == this;
     }
  private:
  B* forward;
  };
  
  class B {
  public:
  B() { backward = 0; }
  attach(A* a) { backward = a; }
  bool invariant() {
     return !backward || backward->forward == this;
     }
  private:
  A* backward;
  };