foo.yaml

  thing:
    seq:
    - id: first
      type: u4
    - id: items
      type: item(_index)
      repeat: eos
    types:
      item:
        params:
          - id: i
            type: u4
        seq:
        - id: delta
          type: u8
        instances:
          base:
            value: i > 0 ? _parent.items[i-1].true_value : _parent.first
          true_value:
            value: base + delta