[samwalrus]

The thing is findall/3 is not really in the standard pure part of Prolog. It has the form: findall(Template,Enumerator,Instances), and can be read as: `Instances is the sequence of instances of Template which correspond to proofs of Enumerator in the order which they are found by the Prolog system" (Craft of prolog) This reading is meta logical because it depends on the instantiation of variables in Template and Enumerator and on the proof strategy of the Prolog system. So for example if you used a meta interpreter to change the search strategy of Prolog from topdown to bottom up, then the result of a call to findall would change. setof/3 does not have this problem because it finds the ordered set and can fail (in contrast to findall which will return an empty list) You need to bear in mind we as programmers are concerned with answers to a query but Prolog returns proofs... I now try and avoid findall/3 in my programs and only use it when I am doing input and output from my core program.