operation AddComment
    inputs: Comment, AnsweredQuestion, AnsweredTest;
    outputs: GradedQuestion;
    precondition: (*
		   * comment cannot equal null
                   *)
		  ( comment != null )
			
			and

		  (*
		   * old and new comment are not same
		   *)
		  ( oldComment != newComment )
		  
			and

		  (*
		   * question cannot equal null
                   *)
		  ( question != null )

			and

		  (*
		   * test cannot equal null
		   *)
		  ( test != null);

    postcondition:(*
		   * a question with a new comment is outputed if and
		   * only if the comment is not equal to the old comment
		   * otherwise the comment is empty and the comment is 
		   * on the question of a test
		   *)
		  ( exist (newComment in gradedQuestion) 
		  	  (gradedQuestion in gradedTest))
		    iff (oldcomment ! = newCOmment);

    description: (*
        Add a given comment to an answer of a student's question.
	The comment will appear in a text box below the question.
    *);
end;

operation DeleteComment
    inputs: Comment, GradedQuestion, GradedTest;
    outputs: GradedTest;
    precondition: (* 
		   * comment cannot equal null
		   *)
		  ( comment != null)

			and
		  
		  (*
		   * comment must be in gradedQuestion
		   *)
		  ( Comment in GradedQuestion )

			and		  

		  (*
		   * gradedTest cannont equal null
		   *)
		  ( gradedTest != null);

    postcondition:(*
		   * an new gradedQuestion with the comment deleteted if and only
		   * if the comment is on the gradedQuestion
		   *)
		  (  question in test );

    description: (*
        Delete the selected a question on a test by the user.	After 
	user comfirms that the question is removed from that student's
	test or the entire class, the test properties are then adjusted.
    *);
end;

operation DeleteQuestion
    inputs: AnsweredQuestion, AnsweredTest;
    outputs: GradedTest;
    precondition: (* 
		   * question cannot equal null
		   *)
		  ( question != null)

			and
		  
		  (*
		   * question must be in test
		   *)
		  ( question in test )

			and		  

		  (*
		   * test cannont equal null
		   *)
		  ( test != null);

    postcondition:(*
		   * an new test with the question deleteted if and only
		   * if the question is on the test
		   *)
		  (  question in test );

    description: (*
        Delete the selected a question on a test by the user.	After 
	user comfirms that the question is removed from that student's
	test or the entire class, the test properties are then adjusted.
    *);
end;

operation ChangeAnswer
    inputs: CorrectAnswer, AnsweredQuestion;
    outputs: GradedQuestion;
    precondition: (* 
		   * correctAnswer cannont equal null
		   *)
		  ( correct != null)

			and

		  (*
		   * newCorrectAnswer cannont equal oldCorrectAnswer
		   *)
		  ( newCorrectAnswer != oldCorrectAnswer )

			and

		  (*
		   * question cannont equal null
		   *)
		  ( question !=null );
    
    postcondition:(*
		   * question is output with the new answer if and only if
		   * the old correct answer to be replaced is not equal to the 
		   * new answer added in the question.
		   *)
		  ( exist (correctAnswer in question) iff (newAnswer != oldAnswer));

    description: (*
        Change the old correct answer to a new answer so the total grade earned
 	by a student or throughout the whole class. The user will declare 
	whether the change is affected to specific student or to the whole
	class. The new correct answer cannot be the same as the old correct
	answer.
    *);
end;

operation ChangeDifficulty
    inputs: Difficulty, AnsweredQuestion;
    outputs: GradedQuestion;
    precondition: (* 
		   * difficulty cannot equal null
		   *)
		  ( difficulty != null )

			and

		  (*
		   * old difficulty cannot equal new difficulty
		   *)
		  ( oldDifficulty != newDifficulty )

			and

		  (*
		   * question cannot equal null
		   *)
		  ( question !=null );
    
    postcondition:(*
		   * question is outputed with new difficulty if and only if
		   * the old difficulty to replaced is not equal to the new 
		   * difficulty added in the question.
		   *)
		  ( exist (difficulty in question) iff (newDifficulty != oldDifficulty));

    description: (*
        Change the old difficulty to a new difficulty range 1-10. The user
	selects a question depending on the success rate. The question is then 
	changed for the entire class that took the test.
    *);
end;

operation ChangePointValue
    inputs: Points, AnsweredQuestion;
    outputs:  GradedQuestion;
    precondition: (* 
		   * points cannot equal null
		   *)
		  ( points != null )

			and

		  (*
		   * old points cannont equal new points
		   *)
		  ( oldPoints != newPoints )

			and
		
		  (*
		   * question cannot equal null
		   *)
		  ( question != null );

    postcondition:(*
		   * a question is outputed with the new point value if and
		   * only if the old point value to be replaced is not equal 
		   * to the new point value added in the question.
		   *)
		  ( exist (points in question) iff (newPoint != oldPoint));

    description: (*
        Change the old point value to a new point value.  The user selects
	a question and depending on the success rate or difficulty changes
	the point value.
    *);
end;