Constraint propagation

Hello, I have a question about constraint propagation in gprolog 1.2.16 
(under linux). Let's consider the following query:

A #<=> (X #\/ Y),
B #<=> (X #\/ Y),
A #<=> 1.

for this query the answer of gprolog is:

A = 1
B = _#81(0..1)
X = _#19(0..1)
Y = _#38(0..1)

but I would expect that A and B reference the same variable, and then we 
should obtain the following result:

A = 1
B = 1
X = _#19(0..1)
Y = _#38(0..1)


why it is not like this ?

Thanks,

Julien Prados


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Julien Prados      Tel:+41 22 37 91083
CUI - University of Geneva
24 rue General Dufour
1211 GENEVE 4/Switzerland