Is there a way to get array substream?

File contains some count of fixed size blocks
Data entry may consist of 1..n block, last block marked with magic signature

The objective is:
step 1: Use array & repeat-until to locate data entries
step 2: Create instance and if necessary, substream to handle variable size data

Is it possible to get array io object?
Or is there a way to handle structure other way (mb pos?)

```
seq:
  - id: entries
    type: entry_t
    repeat: eos
    
types:
  entry_t:
    seq:
      - id: blocks
        type: block_simplified
        size: 1024
        repeat: until
        repeat-until: _.sign == 'EndMarker____'
        
    instances:
      data_processed:
        io: blocks._io   # if it would be possible to get io of all array this will work
		type: entry_ext_t
        size: 1024 * blocks.size # specify size to create substream

  block_simplified: # simple block description just to find end marker
    seq:
      - id: filler
        size: 1024 - (13)
      - id: sign
        type: str
        size: 13

  entry_ext_t: # ext blocks description to handle data
    seq:
   # ....... struct fields - data will be handled here
```

