template< class E >
  class nil : public list< E > { 
  nil  
  public:
  nil() {}
  bool empty() { return 1; }
  E head() { require( false ); return E(); }
  list< E >* tail() { require( 0 ); return 0; }
  bool operator==(list* m) { return m->empty(); }
  };
  
  template< class E >
  class cons : public list< E > { 
  cons  
  public:
  cons(E e, list* l) : _e(e), next(l) {}
  ~cons() { delete next; }
  bool empty() { return 0; }
  E head() { return _e; }
  list* tail() { return next; }
  bool operator==(list* m);
  protected:
  E _e;
  list* next;
  };