a : Prop
inst✝ : Decidable a
this : ¬a → ¬¬a ↔ ¬¬a
⊢ ¬a → a ↔ a
a : Prop
inst✝ : Decidable a
⊢ ¬a → a ↔ a
have := @imp_not_self (¬a)
no goals
a : Prop
inst✝ : Decidable a
this : ¬a → ¬¬a ↔ ¬¬a
⊢ ¬a → a ↔ a
rwa [not_not] at this