Can't use _index in subtypes?
I think for some reason it doesn't like you using _index in subtypes. Here is my file:
```yaml
seq:
  - id: header
    type: header
  - id: table_1
    type: table_type_1
    
types:
  header:
    seq:
      - id: magic
        contents: [0x10, 0x00, 0x00, 0x00]
  
  table_type_1:
    seq:
      - id: table_header
        type: table_header
      - id: table_body
        type: table_body
        size: 4 + ((table_header.table_header_length - 0x04) * 2)
    types:
      table_header:
        seq:
          - id: table_header_length
            type: u2
          - id: table_end_offset
            type: u2
          - id: table_start_offset
            type: u2
          - id: table_offsets
            type: u2
            repeat: expr
            repeat-expr: (table_header_length - 0x06) / 0x02
      table_body:
        seq:
          - id: header_end
            type: u4
        instances:
          table_data_start:
            pos: _parent.table_header.table_start_offset
            size: 0x04
          table_data:
            pos: _parent.table_header.table_offsets[_index]
            size: 0x04
            repeat: expr
            repeat-expr: (_parent.table_header.table_header_length - 0x06) / 0x02
```
The problem im having is that the 4th last line, pos: _parent.table_header.table_offsets[_index]
doesn't seem to work. I have confirmed that it's the _index thats not working, because if i replace it with a 0 it works fine. I came across this issue: https://github.com/kaitai-io/kaitai_struct/issues/598 which seems to be the same as mine, and it is also in a subtype, so i think that's what the issue is