Backwards sequence from the end of a stream?

[Timmmm]

I'm trying to read a format that has a quite dumb layout. One of its TLV blocks has fields that you must read from the start and the end of the block, and then there's a single field in the middle that is all the left over space. Here's what I've got that works:

  block_vcdata:
    seq:
      - id: start_time
        type: u8
      - id: end_time
        type: u8
      - id: memory_required
        type: u8
      - id: bits_uncompressed_length
        type: vlq_base128_le
      - id: bits_compressed_length
        type: vlq_base128_le
      - id: bits_count
        type: vlq_base128_le
      - id: bits_data
        size: bits_compressed_length.value
      - id: waves_count
        type: vlq_base128_le
      - id: waves_packtype
        type: u1
      - id: waves_data
        size: _parent.block_length - 8 - (_io.pos + 32 + position_length + time_compressed_length)
    # The other fields have to awkwardly be instances because you need to
    # read them backwards from the end. Weird format.
    instances:
      position_data:
        pos: _io.size - 8 - 8 - 8 - time_compressed_length - 8 - position_length
        size: position_length
      position_length:
        pos: _io.size - 8 - 8 - 8 - time_compressed_length - 8
        type: u8
      time_data:
        pos: _io.size - 8 - 8 - 8 - time_compressed_length
        size: time_compressed_length
      time_uncompressed_length:
        pos: _io.size - 8 - 8 - 8
        type: u8
      time_compressed_length:
        pos: _io.size - 8 - 8
        type: u8
      time_count:
        pos: _io.size - 8
        type: u8

As you can see waves_data is "the left over space in the middle". (The block_length - 8 btw is because the length inexplicably includes itself.)

To be honest I'm amazed that this works, what with all the crazy dependencies. But I thought I'd better just ask if there is a better way to do this?

[Timmmm]

Actually I guess I can at least use _io.size instead of _parent.block_length - 8. Still - original question stands!