| ⊢ f ≡ f | DfDfEqvDf | 
Proof:
| 1 | (f ⌢ true) ⌢ true ≡ f ⌢ (true ⌢ true) | |
| 2  |  true ≡true                                         | |
| 3  | (true ⌢ true) ≡true                             | 2, def. of              | 
| 4  | f ⌢ (true ⌢ true) ≡ f ⌢ true              | |
| 5  | (f ⌢ true) ⌢ true ≡ f ⌢ true              | |
| 6  |   f ≡ f                               | 5, def. of              | 
qed