ADT

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

  list* interval( element x, element y ) {
  	list* l = new list;
  	if ( x <= y ) {
  		l->tag = INTERVAL;
  		l->e = x; l->z = y;
  		}
  	else l->tag = NIL;
  	return l;
  	}