anchor.diff:      }
     }
   
  +  Collection map(f(Element element)) {
  +    //TODO(jacobr): Implement this.
  +    throw 'Not implemented yet.';
  +  }
  +
     Collection<Element> filter(bool f(Element element)) {
  -    throw 'Not impl yet. todo(jacobr)';
  +    //TODO(jacobr): Implement this.
  +    throw 'Not implemented yet.';
     }
   
     bool every(bool f(Element element)) {
  -    throw 'Not impl yet. todo(jacobr)';
  +    //TODO(jacobr): Implement this.
  +    throw 'Not implemented yet.';
     }
   
     bool some(bool f(Element element)) {
  
positive.code:     }
    }
  
    Collection<Element> filter(bool f(Element element)) {
      List<Element> output = new List<Element>();
      forEach((Element element) {
  
negative.code:     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);
  