anchor.diff:    }
   }
   
  +/**
  + * Nonempty [LinkedList]
  + */
   class Cons<E> extends _LinkedListBase<E> {
     int _length = null;
   
  +  /// The first element of this
     final E elem;
  +
  +  /// The rest of this - without the first element
     LinkedList<E> tail;
   
     Cons(this.elem, this.tail);
  
positive.code:     _last = cons;
    }
  
    LinkedList<E> build([tail = null]) {
      if (tail == null)
        tail = new Nil<E>();
  
negative.code:   bool moveNext() => false;
  }
  
  /**
   * Empty [LinkedList]
   */
  class Nil<E> extends _LinkedListBase<E> {
    bool get isNil => true;
    bool get isCons => false;
  