Unable to access #each helper variables in child scope

I can't find anything in the documentation that says I should be able to use `../@last` but I also don't understand why I can't, or why it throws an error.
The documentation states:

> Some helpers like #with and #each allow you to dive into nested objects. When you include ../ segments in your path, Handlebars will change back into the parent context.

Template:
```
{{#each outer}}
{{#each inner}}
outer index: {{{../@index}}}
inner index: {{{@index}}}
{{/each}}
{{/each}}
```
with data:
```
{
outer: [{
inner: [{},{},{}]
},{
inner: [{},{},{}]
}]
}
```

Throws error (using http://tryhandlebarsjs.com):

> Error(s): Error: Parse error on line 3: ...outer index: {{{../@index}}}inner inde ----------------------^ Expecting 'ID', got 'DATA'

There is a specific scenario I have where I can't find a way to implement the use case without being able to access the `#each` helper variables from a child scope:
https://stackoverflow.com/questions/67994959/how-to-avoid-trailing-comma-when-defining-json-array-by-nested-enumeration-of-hi

-Steve

Start with the nested #each template and sample data in the issue, then reproduce the parse error using the linked tryhandlebarsjs example. Done means determining whether parent #each data variables should be accessible through ../@index and adding the corresponding behavior or documentation clarification.

Written by the indexing model from the issue text.