I need to parse data differently depend on flag in the middle of struct.
```yaml
directory_rec:
    seq:
      - id: file_name
        size: 11
        type: str
        encoding: ASCII
      - id: attribute
        type: u1
      - id: file_size
        type: u4
```
Depending on `attribute` I need to parse data differently. 
Best guess so far is create instance with different struct and use `pos 0` with substream. But I cannot specify the size because I read data with `repeat: until`.

Any suggestion how I could achieve this?