ADT

  typedef int element;
  
  enum { NIL, CONS };
  
  struct list {
  int tag;
  element e;
  list* next;
  };
  

  list* nil()  { 
  nil  
  list* l = new list; l->tag = NIL; return l;
  }
  
  list* cons( element e, list* l) { 
  cons  
  list* x = new list; 
  x->tag = CONS; x->e = e; x->next = l;
  return x;
  }