failTac.lean:2:2-2:6: error: tactic 'fail' failed
a b : Nat
⊢ False
failTac.lean:5:2-5:18: error: giving up
a b : Nat
⊢ False
failTac.lean:14:2-14:21: error: failing here
case left
a b : Nat
⊢ True

case right
a b : Nat
⊢ False