The Logic Book, M. Bergmann, J. Moor, J. Nelson, McGraw Hill, 2004, 10.6E, 2(e), p. 572

Show that the argument is valid:

1. a = b
2. ∴Ka ∨¬ Kb
3. * Kb ......... ACP
4. * Ka .......... 1,3 Id
5. Kb ⊃Ka ......... 3-4 CP
6. ¬ Kb ∨ Ka ......... 5 MI
7. Ka ∨¬ Kb ......... 6 Comm.