internal Fact(string expression, string meaning, ReviewTime reviewTime)
 {
     _reviewTime = reviewTime;
     Expression = expression;
     Meaning = meaning;
     _reviewTime = reviewTime;
 }
 public void GiveAnswer(Answer answer)
 {
     if (answer == Answer.Correct)
     {
     //                NextReview = DateTime.Now.Add(DateTime.Now - LastReview);
     //                LastReview = DateTime.Now;
         _reviewTime = _reviewTime.CalculateNextReview();
     }
 }