doIssue.lean:3:2-3:3: error: Type mismatch
  x
has type
  Nat
but is expected to have type
  IO Unit
doIssue.lean:11:2-11:13: error: Type mismatch
  xs.set! 0 1
has type
  Array Nat
but is expected to have type
  IO Unit
doIssue.lean:19:7-19:20: error: Application type mismatch: The argument
  xs.set! 0 1
has type
  Array Nat
but is expected to have type
  Unit
in the application
  pure (xs.set! 0 1)