public LessonStage CreateLessonStage()
        {
            LessonStage lessonStage = new LessonStage(m_ShapeActionFactory);

            m_LessonStages.Add(lessonStage);
            lessonStage.SetNum(m_LessonStages.Count - 1);
            lessonStage.BecameDirty += OnBecameDirty;

            return(lessonStage);
        }