There is a subset of #uxntal that is perfectly reversible and that does not generate any loss in entropy, meaning that a step debugger could step backward in time without any loss of information.

I implemented a stack-effect checker last week and I think I might be able to detects linear routines too. Jumping around, self-modification, load/store opcodes will need to stash the entropy loss in a 3rd stack which will allow reversion of non-linear operations.

https://web.archive.org/web/20171104231932/http://home.pipeline.com/~hbaker1/ForthStack.html