Section DerivableRulesFromAxiomatization1.
Context {L: Language}
{minL: MinimumLanguage L}
{pL: PropositionalLanguage L}
{Gamma: Provable L}
{minAX: MinimumAxiomatization L Gamma}
{ipGamma: IntuitionisticPropositionalLogic L Gamma}.

Lemma contrapositivePN: forall (x y: expr),
|-- (y --> ~~ x) --> (x --> ~~ y).