anchor.diff:  
     E operator[](int index) => _sorted[index];
   
  -  void clear() => _sorted.clear();
  -
  +  @override
     void addAll(Iterable<E> elements) {
       for(var e in elements) {
         this.add(e);
  
positive.code:     _sorted.add(element);
    }
  
  
    bool remove(Object element) => _sorted.remove(element);
  }
  
negative.code:   }
  
    // TODO: binary insert? O(ln) is better than O(n)
    @override
    void add(E element) {
      for(int i = 0; i < _sorted.length; i++) {
        var compare = element.compareTo(_sorted[i]);
  