public EnrolledSubject(PlainText subjectName, Credits credits, ViewableSituation situation, Common.Professor.Professor professor)
            : base(subjectName, credits)
        {
            Contract.Requires(situation != null, "Viewable situation is null!");
            Contract.Requires(professor != null, "Professor is null!");

            Situation = situation;
            Professor = professor;
        }
 public EnrolledSubject(Guid id, Credits credits, ViewableSituation situation, Common.Professor.Professor professor, PlainText subjectName) : base(subjectName, credits)
 {
     this.id        = id;
     this.professor = professor;
 }