ADT

  typedef int element; 
  struct list;
  
  extern list* nil();
  extern list* cons(element e, list* l);
  extern element head(list* l);
  extern list* tail(list* l);
  extern bool equal(list* l, list* m);
  

OOP

  template< class E > 
  class list {
  public:
  list() { }
  virtual ~list() { }
  virtual bool empty() = 0;
  virtual E head()  = 0;
  virtual list<E>* tail() = 0;
  virtual bool operator==(list<E>* m) = 0;
  };