I was terribly confused because I thought this was referring to Post&#x27;s Correspondence Principle: <a href="http:&#x2F;&#x2F;citeseerx.ist.psu.edu&#x2F;viewdoc&#x2F;summary?doi=10.1.1.380.85" rel="nofollow">http:&#x2F;&#x2F;citeseerx.ist.psu.edu&#x2F;viewdoc&#x2F;summary?doi=10.1.1.380....</a><p>This concerns instead the Post Correspondence Problem: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Post_correspondence_problem" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Post_correspondence_problem</a><p>Surprising namespace clash!