13. Plums are some of them wholesome, and some not.

14. All plums are unwholesome.

[See pp. 59, 60]

-----
| |
| x
| |
|--y--|
| |
| x'
| |
-----

__________

Taking y="diligent students"; x="successful";

15. No diligent students are unsuccessful.

16. All diligent students are successful.

17. No students are diligent.

18. There are some diligent, but unsuccessful, students.

19. Some students are diligent.