Allow expression in index and slice expression

[innovate-invent]

It is very useful to allow data to refer to itself.
I propose bracket-specifier = "[" (number / "*") "]" / "[]" be extended to bracket-specifier = "[" (expression / "*" ) "]" / "[]" for indexes.

slice-expression = [number] ":" [number] [ ":" [number] ] be extended to slice-expression = [expression] ":" [expression] [ ":" [expression] ] for slices

The scope of the expression is the parent of the object being sliced, this can be extended with let()

[innovate-invent]

It is occurring to me that this would be ambiguous with multiselect list.
for example is [a] equivalent to [@.a] (list) or @[a] (index)
currently it is a multiselect list

I was considering [&a] to disambiguate, but that is already defined as a multiselect list of expressions

perhaps a index(@, i) function? but that would duplicate the index operator

perhaps continue with the way a filter disambiguates? something like [! a] or [* a]