I am not an expert in either tag systems (or `find`), but this seems right. Using backreferences to handle copies sounds right (it does add expressive power to

I am not an expert in either tag systems (or `find`), but this seems right. Using backreferences to handle copies sounds right (it does add expressive power to regular expressions but does not give Turing completeness, afaik). I think your first example is missing the "any word of length < 2 is a halting word" condition but it is present in your second example.