Variable in nested shortcode via .Parent

do you have any examples on how to use variables from parent shortcode in a nested shortcode

i tried:

tree.html

{{ $foo := 15 }}
<div>{{ .Inner }}</div>

and leaf.html

<div>{{ with .Parent }}{{ $foo }}{{ end }}</div>

or

<div>{{ .Parent.Get "$foo" }}</div>

or

<div>{{  index .Parent "$foo" }}</div>

None worked

Only option I got working was to define param as an attribute, with leaf.html

<div>{{ .Parent.Get "foo" }}</div>

{{% tree foo="15" %}}
{{% leaf %}}
{{% /tree %}}

You can get anything from the context passed to the parent, but not vars you have defined in the parent.

Any alternative suggestion here, can I set a property in the context of the parent? Which can be read on child?

Bit of context here, on a bootstrap accordion each item references the id of the parent, Id of parent should be unique on page

Is there any way you can use .Ordinal?

Oh, that is relevant indeed, thanx

.Parent.Ordinal is what I used, and works great