Re: A question (bug?) on FD solver

See answer I post in users-prolog

Xiaohua Kong wrote:
> 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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