class LinkedListBuilder<E> {
 LinkedList<E> _first = null;
 Cons<E> _last = null;

 void add(E x) {
   Cons<E> cons = new Cons<E>(x, null);
   if (_first == null) {
     _first = cons;
   } else {
     _last.tail = cons;
   }
   _last = cons;
 }

 LinkedList<E> build([tail = null]) {
   if (tail == null)
     tail = new Nil<E>();
   if (_first == null) {
     return tail;
   } else {
     _last.tail = tail;
     return _first;
   }
 }
}