doIssue.lean:2:2-2:3: error: Type mismatch
  x
has type
  Nat
but is expected to have type
  IO PUnit
doIssue.lean:10:2-10:13: error: Type mismatch
  xs.set! 0 1
has type
  Array Nat
but is expected to have type
  IO PUnit
doIssue.lean:18:7-18:20: error: Application type mismatch: The argument
  xs.set! 0 1
has type
  Array Nat
but is expected to have type
  PUnit
in the application
  pure (xs.set! 0 1)