Nested bag can't be parsed

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Based on `bags.robot` tests
```
*** Test Cases ***
Bag with nested bag
Bag message client
u8 element 1
u8 element2 42
u8 element3 1
u8 first 12
u8 second 55
Client sends message
Bag message server with minimums with nested bag
${msg}= Server receives message # Unable to decode bag value.

*** Keywords ***
Bag message client
New message ConditionalExample Example header:messageType:0xb0b0

Bag message server with minimums with nested bag
New message ConditionalExample Example header:messageType:0xb0b0
Start bag intBag
case 1 u8 foo 42
case 1-2 u8 bar 1
case * Nested bag
End bag

Nested bag
Start bag intBag
case 1 u8 first
case 1 u8 second 55
End bag
```

```
==============================================================================
Tests
==============================================================================
Tests.Test
==============================================================================
Bag with nested bag | FAIL |
Unable to decode bag value.
------------------------------------------------------------------------------
Tests.Test | FAIL |
1 test, 0 passed, 1 failed
==============================================================================
Tests | FAIL |
1 test, 0 passed, 1 failed
==============================================================================
```

Start with the nested-bag reproduction in bags.robot and run the failing test. Trace how the nested bag value is decoded; done means the example passes without the reported "Unable to decode bag value" failure.

Written by the indexing model from the issue text.