The Logic Book, M. Bergmann, J. Moor, J. Nelson, McGraw Hill, 2004, 10.6E, 1(a), p. 571

Show that the following: a = b ⊃ b = a is a theorem:

1. * a = b ......... ACP
2. * b = a ......... Id
3. a = b ⊃ b = a ......... 1-3 CP