Positional Binding - issue using array constructor to generate array of arrays in result

[oatybiscuit]

I use positional binding a lot in mapping arrays, eg foo#$bar.{"value": $, "index": $bar}. In some recent code, generating an array within the mapping iteration I wanted the final result to be an array of arrays, thus used an array constructor '[]' after the '.' mapping, as in array.[$] which generates an array of arrays [ [], [], [] ] as expected.

Adding a positional binding variable so as to be able to use the mapping positional index, the array of array collapses, and the array constructor no longer operates.

1. Should the use of a positional binding at a mapping cause a failure of a post-mapping array constructor in this way?
2. Why (after a great deal of experimentation) does the use of [[ ]] in place of [ ] cause the expected array of results to return?

https://try.jsonata.org/SJItcKMKA

Interestingly I have now learnt that array.[[$]] returns an array of array of arrays... There seems no end to the fun that can be had from JSONata!