asserta and assertz are different???

[Thorsten Krebs]

Hi,

I am new to GNU Prolog and want to use the C interface for constraint
solving in Prolog. But I have a lot of problems with the sematics of Prolog...

I tried to get a little ample program running which manages a database eith
ancestors. Entries are of the form "parent(bob, mary).". The queries are
realized by "anc(X,Y):-parent(X,Y)." and "anc(X,Z):-parent(X,Y),anc(Y,Z).".

I can insert knowledge by adding (asserting) parents and I also can send
queries. No problem so far!

But when I try to delete parents using retract I see VERY deffierent
behaviour wether I used asserta or assertz. Using assertz the only parent left 
is
the one I wanted to delete and using asserta my program crashes at the same
step it "worked" before... (???)

Since I don't know Prolog I would really appreciate any help consideing
assert (differences between asserta and assertz) and retract (does this not
delete?)...

Thanks for any answers,

Greetings,
Thorsten Krebs

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