Yes I agree. Still, the key insight of constraints like dif/2 may also be applicable in this concrete case to limit the candidates already before and even durin

Yes I agree. Still, the key insight of constraints like dif/2 may also be applicable in this concrete case to limit the candidates already before and even during the search. It is this automatic pruning that makes logic programming with constraints so fast for many combinatorial applications. You are right of course that it may end up in brute force though, if there is nothing to prune.