anchor.diff:      _read().forEach(f);
     }
   
  +  Collection map(f(String element)) {
  +    return _read().map(f);
  +  }
  +
     Collection<String> filter(bool f(String element)) {
       return _read().filter(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)) {
      //TODO(jacobr): Implement this.
      throw 'Not implemented yet.';
    }
  
    Collection<Element> filter(bool f(Element element)) {
      //TODO(jacobr): Implement this.
      throw 'Not implemented yet.';
    }
  
    bool every(bool f(Element element)) {
      //TODO(jacobr): Implement this.
      throw 'Not implemented yet.';
    }
  
    bool some(bool f(Element element)) {
  