Pop-2 was a programming language initially developed in the late 1960s at the University of Edinburgh as a general purpose programming language especially suitable for Artificial Intelligence programming.
Pop-11, now part of the Poplog system, is still recognisably a descendant of Pop-2.
Because Poplog is still in use, it is easy to get the source code and documentation for Poplog. However, if you want to understand surviving code in Pop-2, like Max Bramer's RMYCIN system, you want Pop-2 documentation.
This is all the Pop-2 documentation I have been able to find. If anyone knows of other documentation, I would love to add it to this list. I would particularly like to see "Pop-2 in Pop-2" and WonderPop.
These two reports are both copyright 1976 D. J. M. Davies, the Department of Computer Science, University of Western Ontario, London, Ontario, Canada, and are provided here by permission. Thanks to Dianne McFadzean who located and copied them, and the Department Chair, who gave permission.
The Silver Book was published at a point where a 'while' loop (using the keyword 'loopif'; this was before 'while' had become generally popular) had been added to the language, but the library had not yet been revised to use it, so the library makes much use of labels and gotos. As far as I am aware, all of the library files can easily be modified to use loops instead of gotos. Indeed, as a WonderPop user, I had completely forgotten that there was a goto in the language.
The influence of Pop-2 has been more than you might imagine. Traces of Pop-2 syntax can still be found in Standard ML, in Prolog, and in Erlang.