What exactly do you try to achieve?
Perhaps I did not pick the best wording. I am thinking around this example Is it possible to remove duplicate wikitext code when showing both codeblocks and the result of rendering them? - #2 by RolandH where the transclusion widget is called with two parameters: definitions and demoCode. I am wondering if it is possible to rewrite this to accept just one parameter - the whole code block - and parse it into definitions and demoCode inside the template code. These are two distinct pieces of a string - the header is the definitions and the footer is the actual code that uses them. But I don’t understand how to find where the definitions part ends.