Re: Boundary conditions

>        uniques_list(Control, [X|Check], Unique0, Unique) :-

>            \+ member(X, Control),
>            !, % you need this so the next clause isn't tried.
>            Unique0 = [X|Unique1],
>            uniques_list(Control, Check, Unique1, Unique).
>
>*The theory (and Alan Baljeu) say that the cut is needed here but even
>without the !, my procedure now works fine. Is this just plain luck?*

To illustrate that it doesn't work fine without the cut, try this query:

?- uniques_list([1,2,3,4], [4,5,6], Unique), length(Unique, 1).

You expect a list of 2 elements, but this gives a list of 1.  This is because your program, without the cut, has multiple potential solutions, and Prolog will try each of them until something succeeds.
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