I personally believe all this is evident and obvious, but... I can't understand why you're telling me! I knew. Still stands the point I tried to make to the OP: namely, that he should carefully choose the frameset for the problem. He reasoned in terms of sets: that these are "implemented" (please bear with me being slightly sloppy wrt terminology) with lists in the referenced module was considered a side effect to be used afterwards, which suggested him an overkill of an algorithm, while if he had resoned in terms of lists in the first place, he would have probably chosen a saner one to start with.