I did not say he or anyone could not ask for the source code on this site.
There are many places on this site where published books are noted, E.g.
Useful Prolog references, and some may have links to the source with them. Sometimes the links to the source code become broken, sometimes the links to the source code are to illegal copies of the source. If the OP knew of such a link on this site then I wanted to verify that it was not to an illegal copy.
Before posting my reply I did look for the source for the book. First looked at the WorldCat page for the book to find the publisher
Publisher:Adisson Wesley, Harlow [ect.], 2012
Then checked the Adisson Wesley publisher site and did not find the book.
Using a Google search found the book on the Pearson site:
Prolog programming for artificial intelligence
and checked that page and found no place to download the source code.
Then posted the reply.
To make this more clear.
If this site, the SWI-Prolog Discourse forum, contains a link to the source code for Prolog Programming for Artificial Intelligence (4th Edition) by Ivan Bratko then please let me know so that I check if the link is to a legal copy. If the link is to an illegal copy I will edit the post to remove the link.
EDIT
In looking at the Pearson site in more detail noticed that I overlooked
Clicking I'm an educator then Instructor resources (link) might allow one to download the code if they sign up. As I do not intend to sign up I can not say more.
Thanks @Boris for replying, it may be take a closer look.