A qestion (bug?) of FD solver

Hi,

I am using FD solver to work on some verification problem that is modeled as CSP problem. Since most constraints are linear, I choose partial consistency operators in FD solver.
Here is a small example:

========================
ieq(LD1,LD2,Va):-
LD1 = [X1,X2,X3,X4],
LD2 = [D1,D2],
fd_domain(LD1,0,20),
fd_domain(LD2,1,12),
Va #=<5,
Va #>=0,
X1 #= 0,
X2 #=< X1+4,
X2 #>= X1+2,
X3 #= X1+D1,
X4 #= X2+D2,
D1 #=< D2+Va,
D1 #>= D2-Va,
X4 #=< X3.
===========================
By asking ieq(LD1,LD2,Va).
The solver gives answer:
LD1 = [0,_#25(2..4),_#47(3..12),_#69(3..12)]
LD2 = [_#91(3..12),_#113(1..10)]
Va = _#134(0..5)

However, there is no assignment exsits if Va=0. 
Actually, labeling Va will return give answer that Va could b [2,5].
==============================================

Several questions on this penominon:
- Is there any problem with the code? Or it is a problem that caused by the algrithm.
- Without labeling, if the solver gives answer "yes" and return a domain, does that means there exist at least one solution? (Even though the answer of the variable domains is not accurate).
- The computation cost of labeling.

Thanks
Xiaohua Kong

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