Is it for instances possible to retrieve the instance index?
I'm trying to write a .ksy for MRP message types (IEEE 802-1Q, section 10.8)

Without going into to much detail, it tries to save data by packing multiple values (2 bit values f.e.) into a single byte. The  number of values are specified in a separate field.

Say for example that all values are '1'. The data would look like:

```
number of values = 1
values = 0x40       // [ '1' ]

number of values = 5
values = 0x01 0x55  // [ '1 ', '1' , '1', '1', '1']
```

Currently, my .ksy contains:

```
- id: number_of_values
  type: u1
- id: values
  size: (number_of_values + 3) / 4
```

This will correctly get the values as a byte stream, but I want to be able to parse them to a list of values.
Is there a way to do this? One way would be to use 'instances', but then I would need access to the index of the instance. Looking at the code, this doesn't seem possible (yet).

