public static CoursesCounter Random()
        {
            var existingCourses = ListMother <CourseId> .Random(CourseIdMother.Random);

            return(Create(
                       CoursesCounterIdMother.Random(),
                       CoursesCounterTotalMother.Create(existingCourses.Count),
                       existingCourses
                       ));
        }
        public static CoursesCounter Incrementing(
            CoursesCounter existingCounter, CourseId courseId)
        {
            var existingCourses = new List <CourseId>(existingCounter.ExistingCourses);

            existingCourses.Add(courseId);

            return(Create(
                       existingCounter.Id,
                       CoursesCounterTotalMother.Create(existingCounter.Total.Value + 1),
                       existingCourses
                       ));
        }