Creating a counter
I was wondering if there was a good way to create a counter. I'm not sure of a good way to describe this, so I'll just go with the example I'm trying to work on. I'm attempting to create a file format parser for the [U3D File Format](https://www.ecma-international.org/publications/standards/Ecma-363.htm). 

Specifically, I'm attempting to handle the Stay or Move field described in 9.6.1.3.4.7 (page 72) which is in the resolution_update type. The Stay or Move field is repeated based on the number of times that the face_orientation in new_face_position_info is 0x01. 

Following is my template in its current state, omitting all but the relevant parts:

```yaml
  resolution_update:
    seq:
      - id: split_position_index
        type: u4
      - id: new_diffuse_color_info
        type: new_diffuse_color_info
      - id: new_specular_color_info
        type: new_specular_color_info
      - id: new_texture_coord_info
        type: new_texture_coord_info
      - id: new_face_count
        type: u4
      - id: new_face_position_info
        type: new_face_position_info
        repeat: expr
        repeat-expr: new_face_count
      - id: stay_or_move
        type: u1
  new_face_position_info:
    seq:
      - id: shading_id
        type: u4
      - id: face_orientation
        type: u1
        enum: face_orientation
      - id: third_position_type
        type: u1
        enum: third_position_type
      - id: local_third_position_index
        type: u4
        if: third_position_type == third_position_type::local_third_position_index
      - id: global_third_position_index
        type: u4
        if: third_position_type == third_position_type::global_third_position_index
```

I can't think of a good way to do this. If there isn't one, can some sort of counter to Kaitai Struct?