Inheritance: BaseModel
 public override bool CanModify(User user)
 {
     return IsValid && Id == user.Id;
 }
 public override bool CanModify(User user)
 {
     throw new NotImplementedException("Need the ToDoList as well.");
 }
 public bool CanModify(User user, ToDoList toDoList)
 {
     return IsValid && BelongsToList(toDoList) && toDoList.CanModify(user);
 }
 public virtual bool CanModify(User user)
 {
     return true;
 }