a b : ℕ,
a_1 : a ≠ b,
H : a = b
⊢ false
-------
a b : ℕ,
a_1 : ¬¬a = b,
H : ¬a = b
⊢ false
-------
by_contradiction.lean:22:3: error: tactic by_contradiction failed, target is not a negation nor a decidable proposition (remark: when 'local attribute classical.prop_decidable [instance]' is used all propositions are decidable)
state:
p q : Prop,
a : ¬¬p
⊢ p
-------
p q : Prop,
a : ¬¬p,
H : ¬p
⊢ false