I would say that he has an enormous amount of experience with several languages. He lists on his webpage: "Programming languages used over the past 39-years include C/C++, Common Lisp/CLOS/MOP (and other Lisp varients), ALGOL, FORTRAN, PL/1, SNOBOL, numerous assembly languages, and others."<p>So, he probably could have written it in C++ (or SNOBOL ;-) ), but he chose to do it in Lisp.