be-linking supports an option to pass a value around in a downward direction:
<host-element>
#shadow
<input type=number be-linked='
On input event of $0
pass value property as number
to slide index property
of slide show id
within root node.
'>
...
<my-carousel id=slide-show></my-carousel>
</host-element>
be-passing supports this scenario with more compact notation:
<host-element>
#shadow
<input type=number be-passing='of value to #slide-show on input.'>
...
<my-carousel id=slide-show></my-carousel>
</host-element>
<host-element>
#shadow
<input type=number be-passing='of value to my-carousel on input.'>
...
<my-carousel></my-carousel>
</host-element>