One note of interest about pg's source code is that the dot functions (and., pair., etc...) are based in the host Lisp and not `eval.`. The implications are th

One note of interest about pg's source code is that the dot functions (and., pair., etc...) are based in the host Lisp and not `eval.`. The implications are that pg's functions can take advantage of lexical scope, whereas McCarthy's original Lisp was dynamically scoped. This will make a huge difference if you are so inclined to use pg's code to do some hardcore Lisp anthropology. I did so a few years back using pg's code and it was a blast. I then wrote a small Lisp runtime with dynamic scope (http://github.com/fogus/lithp http://github.com/fogus/lithp) to do the same exercise and it turned out to be a very different experience. It's worth trying it out to gain a full appreciation for the tools the Lisp pioneer's had to work with. Thanks for linking this article -- it's one of my favorites.