|Fig. 14. A protected subsystem to implement the grade-keeping system described in the text. P1, which can be invoked by all students in the subjoct, is programmed to return the caller's grade for a particular assignment or the distribution of all grades for an assignment. P2 which can be invoked by the teaching assistants for the subject, is programmed to allow the addition of new grades to the record but to prevent changing a grade once it is entered. P3, which can be invoked only by the instructor, is programmed to read or write on request any data in the grade record.