code: @NonNull
      @Override
      public ObservableTransformer<ParseGroupsOp, List<Card>> getGroupTransformer() {
          return new ObservableTransformer<ParseGroupsOp, List<Card>>() {
              @Override
              public ObservableSource<List<Card>> apply(Observable<ParseGroupsOp> upstream) {
                  return upstream.map(new Function<ParseGroupsOp, List<Card>>() {
                      @Override
                      public List<Card> apply(ParseGroupsOp parseGroupsOp) throws Exception {
                          return parseGroup(parseGroupsOp.getArg1(), parseGroupsOp.getArg2());
                      }
                  });
              }
          };
      }
docs: {@inheritDoc}