Error (warning 16 [unerasable-optional-argument]): this optional argument cannot be erased.

```
File "src/parmap.ml", line 105, characters 15-73:
105 | let redirect ?(path = (Printf.sprintf "/tmp/.parmap.%d" (Unix.getpid ()))) ~id =
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error (warning 16 [unerasable-optional-argument]): this optional argument cannot be erased.
```

Version: 1.2.3
ocaml-4.12.0
FreeBSD 13