| true : bool
| false : bool.
-(* destruct does not work *)
theorem not_eq_true_false : true ≠ false.
-@nmk #Heq change with match true with [true ⇒ False|false ⇒ True]
->Heq // qed.
+@nmk #Heq destruct
+qed.
definition notb : bool → bool ≝
λ b:bool. match b with [true ⇒ false|false ⇒ true ].