anchor.diff:        throw new UnimplementedError('Excluded paths are not supported yet');
       }
       Set<Folder> excludedFolders = new HashSet<Folder>();
  -    // diff
  -    Set<Folder> currentFolders = _contexts.keys.toSet();
  -    Set<Folder> newFolders = new HashSet<Folder>();
  -    Set<Folder> oldFolders = new HashSet<Folder>();
  -    for (Folder currentFolder in currentFolders) {
  +    // destroy old contexts
  +    for (Folder contextFolder in contextFolders) {
         bool isIncluded = includedFolders.any((folder) {
  -        return folder.contains(currentFolder.path);
  +        return folder.contains(contextFolder.path);
         });
         if (!isIncluded) {
  -        oldFolders.add(currentFolder);
  -      }
  -    }
  +        _destroyContext(contextFolder);
  +      }
  +    }
  +    // create new contexts
       for (Folder includedFolder in includedFolders) {
  -      bool wasIncluded = currentFolders.any((folder) {
  +      bool wasIncluded = contextFolders.any((folder) {
           return folder.contains(includedFolder.path);
         });
         if (!wasIncluded) {
  -        newFolders.add(includedFolder);
  -      }
  -    }
  -    // destroy old contexts
  -    for (Folder folder in oldFolders) {
  -      _destroyContext(folder);
  -    }
  -    // create new contexts
  -    for (Folder folder in newFolders) {
  -      _createContexts(folder, false);
  +        _createContexts(includedFolder, false);
  +      }
       }
     }
   
  
positive.code:    * determine the context directories.
     */
    void setRoots(List<String> includedPaths, List<String> excludedPaths) {
      // included
      Set<Folder> includedFolders = new HashSet<Folder>();
      for (int i = 0; i < includedPaths.length; i++) {
  
negative.code:    *
     * If [folder] itself contains a 'pubspec.yaml' file, subfolders are ignored.
     *
     * If [withPubspecOnly] is `true`, a context will be created only if there
     * is a 'pubspec.yaml' file in [folder].
     *
     * Returns create pubspec-based contexts.
     */
    List<_ContextInfo> _createContexts(Folder folder, bool withPubspecOnly) {
  