There's a great 1986 book "Designing and Programming Personal Expert Systems" by Feucht and Townsend that implements expert systems in Forth (and in the process, much of the capability of Prolog and Lisp).
I assisted a graduate student a long time ago in his implementation of the Warren Machine for compiling Prolog. At its core it was essentially a threaded interpreter.
Thinking about the amount of thought and energy that went into this, back in 1987 -- mostly preinternet, pre-AI. Damn.
I feel really lucky that we get to build on things like this.