anchor.diff:      return last;
     }
   
  +  Collection map(f(Element element)) => _filtered.map(f);
     Collection<Element> filter(bool f(Element element)) => _filtered.filter(f);
     bool every(bool f(Element element)) => _filtered.every(f);
     bool some(bool f(Element element)) => _filtered.some(f);
  
positive.code:   }
  
    void forEach(void f(Node element)) => _toList().forEach(f);
  
    Collection<Node> filter(bool f(Node element)) {
      List<Node> output = new List<Node>();
  
negative.code:     }
    }
  
    Collection map(f(Element element)) {
      List output = new List();
      forEach((Element element) {
        output.add(f(element));
      });
      return output;
    }
  
    Collection<Element> filter(bool f(Element element)) {
      List<Element> output = new List<Element>();
      forEach((Element element) {
  