decideTactic.lean:13:22-13:28: error: tactic 'decide' proved that the proposition
  1 ≠ 1
is false
decideTactic.lean:21:28-21:34: error: tactic 'decide' failed for proposition
  unknownProp
since its 'Decidable' instance reduced to
  Classical.choice ⋯
rather than to the 'isTrue' constructor.