I've been fascinated by concurrent constraint programming (CCP) for a few years now, but it's hard to find source code for any of the system from back then. My reference for most of this topic is Vijay Saraswat's monograph of the same name, and he mentions systems like "Concurrent Prolog", "GHC" (no relation to the Glasgow Haskell Compiler!), and most importantly for me, "Herbrand", the instantiation of CCP over the Herbrand term universe that Saraswat developed.

If anyone is familiar with these topics, any pointers would be much appreciated!