How to dynamically switch type based on a condition?
```ksy
  - id: ar
    type: '_parent.osu_ver < 20140609 ? u1 : f4'
  - id: cs
    type: '_parent.osu_ver < 20140609 ? u1 : f4'
  - id: hpd
    type: '_parent.osu_ver < 20140609 ? u1 : f4'
  - id: od
    type: '_parent.osu_ver < 20140609 ? u1 : f4'
```

This will not work as kaitai parses that as the `type` without checking it. Case does not work (at least in my case) when used to compare values. What else can I do to achieve the same effect?