negative numbers

Hello all,
I know that Gprolog does not handle negative intergers.

However, someone could tell me why gprolog rejects vilently this :

?- fd_domain(I,1,4), fd_element(I,[12, 5, -7,9], X).

Fatal Error: Segmentation Violation

Ok, she does not like my -7.

But with :
X #= -1.
I get just (gently)
no.

PS : the original resuest was :
 ?- fd_domain([S1, S2], 1, 7), S1 #=S2 +X, fd_domain(I,1,4), 
fd_element(I,[-1,-2,1,2], X), fd_labeling([S1,S2]).

I'm trying to get S1 and S2 (2 days of a week) being two consecutive 
days or with a difference of 2 .
An elegant way to this in gprolog (a  short one is wellcome : I'm trying 
to illustratre CLP power in an introduction).

Thanks.
Alex

-- 
Aleksander S. Saidi
Maître de Conférences
Ecole Centrale de Lyon
Département Mathématiques-Informatique
Mél : [email protected]
Tél : 04.72.18.65.30, Fax : 04.78.33.16.15

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