Originally Prolog is not the first priority in programming languages I intended to learn after Lisp. But during reading along the Prolog chapter in On Lisp, I realized that Prolog is really an interesting language. Therefore I downloaded swi-prolog, update Emacs with more powerful prolog mode, and started to follow one Prolog tutorial.
Suddenly I realized that it might be better for me to learn Prolog inside Lisp. The implementation of Prolog presented in On Lisp is already quite powerful (provided the functionality like cut, is, and especially the ability to integrate with Lisp functionalities). Therefore I could try the examples in Prolog tutorials/books both in Prolog (as reference) and Lisp. If there are some Prolog functionalities that are not available in the Lisp implementation, I can try to implement them if possible. Sounds quite interesting.