typedef int element;\fbox{ADT}
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);
template< class E >\fbox{OOP}
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; };