The file halting.sb is the halting-problem demonstration in Scratch (and the
other stuff projected onscreen during lecture) but can't be opened in regular
Scratch or BYOB.  The file BYOB-alpha.image is the alpha-test version with
first-class procedures that you need to examine it.  (It's not really
runnable, of course.)  But the important part, the TRY block's script, is
in a screen capture picture in halting.jpg