public static string GetBooksByCategory(BookShopContext context, string input)
        {
            StringBuilder output = new StringBuilder();
            //Return in a single string the titles of books by a given list of categories.
            //Order by title alphabetically.
            string[] categories = input.Split(" ", StringSplitOptions.RemoveEmptyEntries);

            string[] bookTitles = context
                .Books

                 .Where(book => book.BookCategories.Any(bc =>categories.Contains(bc.Category.Name.ToLower())))

                .Select(book => book.Title)
                .OrderBy(book => book)
                .ToArray();

                foreach(string title in bookTitles)
            {
                output.AppendLine(title);
            }

            return output.ToString().TrimEnd();
        }