Great article! I noticed an issue with the division operator though.
with r s going into result set t.
ALL of set s must match row A with the same in row B.

Here is a modified example from what you had above (made to have an output)

Relation in table 'r' :
Code:
+--------+-------+
|  A     | B     |
+--------+-------+
|   a    |   1   | 
|   b    |   2   |
|   a    |   2   |
|   a    |   3   |
|   p    |   3   |
|   p    |   4   |
+--------+-------+
Relation in table 's' :
Code:
+------+
|  B   |
+------+
|  2   |
|  3   |
+------+
Therefore, r s :
Code:
+------+
|  A   |
+------+
|  a   |
+------+
To reiterate, a is the only result because it is the only tuple in set r that matches every row in set s