1.   α → (¬β → α)                 Ak
2.   α                             ipotesi
3.   ¬β → α                       MP(1,2)
4.   ¬α → (¬β → ¬α)               Ak
5.   ¬α                           ipotesi
6.   ¬β → ¬α                      MP(4,5)
7.   (¬β → ¬α) → ((¬β → α) → β)   A¬
8.   (¬β → α) → β                 MP(6,7)
9.   β                            MP(3,8)