I was terribly confused because I thought this was referring to Post's Correspondence Principle: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.380.85

I was terribly confused because I thought this was referring to Post's Correspondence Principle: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.380.85 http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.380.... This concerns instead the Post Correspondence Problem: https://en.wikipedia.org/wiki/Post_correspondence_problem https://en.wikipedia.org/wiki/Post_correspondence_problem Surprising namespace clash!