Author here: doing this in any language is probably going to depend on some implementation details of the language repl, which may require dropping down to a level where you have access to these internals - for example, I can't think of a way you could achieve it in strict mode JS.

Python may well be able to do it with just the stdlib since it has excellent debugging capabilities, but Tcl is relatively weak. You might be able to do something with `tcl::unsupported::getbytecode` and checking the frame level, but this was just a throw-away novelty so I'm not particularly motivated to find out.