Among the basic Prolog interpreters is the one in the later chapters of our patron Paul Graham's On Lisp, available online for free. Graham writes it with Comm

Among the basic Prolog interpreters is the one in the later chapters of our patron Paul Graham's On Lisp, available online for free. Graham writes it with Common Lisp macros and macro-based continuations all in about 100 expertly described and lovingly documented lines of CL. Very educational, if not so practical.