Instance Classical2GodelDummett: GodelDummettPropositionalLogic L Gamma.

(* Specifications: *)
Lemma contrapositiveNN: forall (x y: expr),
|-- (~~ y --> ~~ x) --> (x --> y).
Lemma contrapositiveNP: forall (x y: expr),
|-- (~~ y --> x) --> (~~ x --> y).