I was terribly confused because I thought this was referring to Post's Correspondence Principle: <a href="http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.380.85" rel="nofollow">http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.380....</a><p>This concerns instead the Post Correspondence Problem: <a href="https://en.wikipedia.org/wiki/Post_correspondence_problem" rel="nofollow">https://en.wikipedia.org/wiki/Post_correspondence_problem</a><p>Surprising namespace clash!
If you find this interesting, take a look at FRACTRAN[1].<p>[1] <a href="https://en.wikipedia.org/wiki/FRACTRAN" rel="nofollow">https://en.wikipedia.org/wiki/FRACTRAN</a>