// CQRS
class ProductService {
  // commands
  commandAddProduct: Subject<Product> = new Subject<Product>();

  // queries
  products$ : Observable<Product[]> = combineLatest(
    this.productApi.list$, // 1
    this.commandAddProduct, // 2
  )
    .pipe(map([
      { data: products } // 1
      add, // 2 // @todo implement splice()
    ]) => products);

  productsWithCategories$ : Observable<Product[]> = combineLatest(
      this.products$, // 1
      this.categoriesService.categories$, // 2
    )
    .pipe(map([
      products, // 1
      { data: categories }, // 2
    ]) => ...someMap...);
}