private static void Example2()
        {
            Chapter c1 = new Chapter(10, "OO");
            Chapter c2 = new Chapter(20, "Abstraction");

            c1.AddPublication(c2);

            Section s1 = new Section(1, "Constructors");
            Section s2 = new Section(2, "Visibility");

            c2.AddPublication(s1);
            c2.AddPublication(s2);

            Chapter c3 = new Chapter(20, "Relationships");

            c1.AddPublication(c3);

            Section s3 = new Section(3, "Associations");

            c3.AddPublication(s3);


            Console.WriteLine($"{c1.Title}[{c1.TotalPages()}]");
            Console.WriteLine($"|--{c2.Title}[{c2.TotalPages()}]");
            Console.WriteLine($"|--|--{s1.Title}[{s1.TotalPages()}]");
            Console.WriteLine($"|--|--{s2.Title}[{s2.TotalPages()}]");
            Console.WriteLine($"|---{c3.Title}[{c3.TotalPages()}]");
            Console.WriteLine($"|--|--{s3.Title}[{s3.TotalPages()}]");
        }