As an example, let's say I make some small modifications to how my TW displays tags by overriding shadow tiddler $:/core/ui/TagTemplate
I then want to display the difference using the original and my modification using the (new) diff text widget. I would want to write something like:
<$diff-text source=??? dest={{$:/core/ui/TagTemplate}}/>
where ??? would be an expression that could retrieve the TW-built-in unaltered content of the shadow tiddler