🚀 go-pugleaf

Not having much experience in Prolog programming I have one question which
seems very difficult to me:

Can you program a IF...THEN...ELSE in Prolog without using the cut?

Let me explain my problem in a few words:
I have a predicate A/3 containing some tuple (X,Y,Z), where Z maybe 0 or 1.
There is a tuple (X,Y,1) for all (X,Y), but there does not have to be a (X,Y,0).
What I want to have is a predicate B/3 containing exactly one tuple (X,Y,U)
for all (X,Y) with U is the lowest Z for which a(X,Y,Z). is true. Or formulated
in another language:
IF (exists a(X,Y,0)) THEN b(X,Y,Z) = a(X,Y,0) ELSE b(X,Y,Z) = a(X,Y,1).
By the way, I need to program this without the cut and without using list data
structures not supported by my test system.

Can anyone help me? Thanks in advance.

Michael
-- 
Michael Meskes, Lehrstuhl fuer angewandte Mathematik insb. Informatik,
RWTH-Aachen, Ahornstr. 55, D-W-5100 Aachen
Email: meskes@ulysses.informatik.rwth-aachen.de