| ?- listing.
| ?- X(baba,wo).
exception: error(syntax_error('user_input:4 (char:2) . or
operator expected after _expression_'),read_term/3)
| ?- x(Y,wo).
It's actually not a bad question, at least not to me because I
can't answer it as well as I'd like. I have been using Prolog in
earnest for about five months every day and I think I know what I
am doing but a question like this can catch you out!
Your first question about "X(baba,wo)" shows that you
may not have grasped the way you ask questions of Prolog in
general which is fine if you are just starting out. If you want
facts that state who is the father you should (as you have) have this:
but then you would phrase the question like this:
I hope now you can begin to see that you are thinking "the
wrong way around" at least when it comes to expressing your
facts about whom is the father of who. That question will cause
prolog to scan the database for facts that match and there is only one.
Hope that helps.