1638.lean:6:3: error: invalid exact tactic, trying to solve goal using itself
state:
2 goals
⊢ true

⊢ true
1638.lean:13:3: error: invalid exact tactic, trying to solve goal using itself
state:
⊢ true
1638.lean:20:3: error: exact tactic failed, failed to assign 
  id ?m_1
to metavariable ?m_1 (possible cause: occurs check failed)
state:
2 goals
⊢ true

⊢ true