Getting $index of grand parent in a nested loop

How to access the index of the grand parent in the nested loop?

For example:

<div class="loop" data-bind="foreach: rows">
    <div class="nested-loop" data-bind="foreach: cells">
        <div class ="nested-nested-loop" data-bind="foreach: candidates, css : {selected : $root.isSelected($parentContext.$parentContext.$index(), $parentContext.$index(), $index())}">
            Candidate index: <span data-bind="text: $index()"></span>
            Cell index: <span data-bind="text: $parentContext.$index()"></span>
            Row index: <span data-bind="text: $parentContext.$parentContext.$index()"></span>
        </div>
    </div>
</div>

I tried to bind like this:

css : {selected : $root.isSelected($parentContext.$parentContext.$index(), $parentContext.$index(), $index())}

and I encountered:

TypeError: $parentContext.$parentContext.$index is not a function