Class(a1): {{t1, t2, t3}, {t4}}
Class(a2): {{t1, t3, t4}, {t2}}
Class(a3): {{t1, t4}, {t2, t3}}

##</code><code>##

After processing Class(a1):

     \  t1  | t2 | t3 | t4
 t1      0  |  0 |  0 |  1
 t2            0 |  0 |  1
 t3                 0 |  1
 t4                      0

After processing Class(a2):

     \  t1  | t2 | t3 | t4
 t1      0  |  1 |  0 |  1
 t2            0 |  1 |  2
 t3                 0 |  1
 t4                      0

After processing Class(a3):

     \  t1  | t2 | t3 | t4
 t1      0  |  2 |  1 |  1
 t2            0 |  1 |  3
 t3                 0 |  2
 t4                      0