docs: public List<Collection> getTree(String collectionId, String userId) throws FlickrException {
          Map<String, Object> parameters = new HashMap<String, Object>();
          parameters.put("method", METHOD_GET_TREE);
          if (collectionId != null) {
              parameters.put("collection_id", collectionId);
          }
          if (userId != null) {
              parameters.put("user_id", userId);
          }
  
          Response response = transportAPI.get(transportAPI.getPath(), parameters, apiKey, sharedSecret);
          if (response.isError()) {
              throw new FlickrException(response.getErrorCode(), response.getErrorMessage());
          }
  
          List<Collection> collections = new ArrayList<Collection>();
          Element mElement = response.getPayload();
  
          NodeList collectionElements = mElement.getElementsByTagName("collection");
          for (int i = 0; i < collectionElements.getLength(); i++) {
              Element element = (Element) collectionElements.item(i);
              collections.add(parseTreeCollection(element));
          }
          return collections;
      }