code: public Observable<List<Product>> getAllProducts() {
      return Observable.zip(getProducts(0), getProducts(1), getProducts(2), getProducts(3),
          (products0, products1, products2, products3) -> {
            List<Product> productList = new ArrayList<Product>();
            productList.addAll(products0);
            productList.addAll(products1);
            productList.addAll(products2);
            productList.addAll(products3);
            return productList;
          });
    }
docs: Get a list with all products from backend