Prolog Ambiguous Output

I'm writing a mini sudoku solver in Prolog. The sudoku consists of 4x4 fields, e.g.

_ _ 2 3
_ _ _ _
_ _ _ _
3 4 _ _

If the sudoku is has only one possible solution (as the sudoku above), my program works as expected. But if I replace at least one given field with a gap (e.g. by replacing 4 by _), the output of Prolog is like this:

[_#3(1:4),_#23(1:4),2,3.....

I guess "_#3(1:4)" means "take 1 or 4", but I want each solution in a separated, non-ambiguous list. This is my code:

sudoku(Puzzle,Solution):-
        Puzzle=Solution,
        fd_domain(Solution,1,4),

        Puzzle=[S11,S12,S13,S14,
            S21,S22,S23,S24,
            S31,S32,S33,S34,
            S41,S42,S43,S44],

        fd_all_different([S11,S12,S13,S14]),
        fd_all_different([S21,S22,S23,S24]),
        fd_all_different([S31,S32,S33,S34]),
        fd_all_different([S41,S42,S43,S44]),

        fd_all_different([S11,S21,S31,S41]),
        fd_all_different([S12,S22,S32,S42]),
        fd_all_different([S13,S23,S33,S43]),
        fd_all_different([S14,S24,S34,S44]),

        fd_all_different([S11,S12,S21,S22]),
        fd_all_different([S13,S14,S23,S24]),
        fd_all_different([S31,S32,S41,S42]),
        fd_all_different([S33,S34,S43,S44]).

When I loaded it into Gnu Prolog, I type the following query:

sudoku([_,_,2,3,_,_,_,_,_,_,_,_,3,_,_,_],Sol).

What am I doing wrong?

your Prolog is advising you of multiple solution, showing the residual constraints in store. You need to label the variables, i.e. let the engine enumerate valid values.

?- sudoku([_,_,2,3,_,_,_,_,_,_,_,_,3,_,_,_],Sol),fd_labeling(Sol).

Sol = [1,4,2,3,2,3,1,4,4,1,3,2,3,2,4,1] ? ;

Sol = [1,4,2,3,2,3,1,4,4,2,3,1,3,1,4,2] ? ;

Sol = [1,4,2,3,2,3,4,1,4,1,3,2,3,2,1,4] ? ;

Sol = [4,1,2,3,2,3,1,4,1,4,3,2,3,2,4,1] ? ;

Sol = [4,1,2,3,2,3,4,1,1,2,3,4,3,4,1,2] ? ;

Sol = [4,1,2,3,2,3,4,1,1,4,3,2,3,2,1,4]