error_pos.lean:1:39: error: type expected at
  B
error_pos.lean:1:8: warning: declaration '_example' uses sorry
error_pos.lean:4:43: error: type expected at
  B
error_pos.lean:9:40: error: type expected at
  B
error_pos.lean:11:36: error: type expected at
  B
error_pos.lean:13:43: error: type expected at
  B
error_pos.lean:15:105: error: invalid let-expression, expression
  a₁
has type
  B
but is expected to have type
  A
error_pos.lean:15:133: error: invalid let-expression, expression
  a₂
has type
  B
but is expected to have type
  A