In this particular example, specifying that N > 0 helps with running the predicate "backwards": You can use the query "length(L, 3)" to enumerate "all" lists of

In this particular example, specifying that N > 0 helps with running the predicate "backwards": You can use the query "length(L, 3)" to enumerate "all" lists of length 3, for example: ?- length(L, 3). L = [_G926, _G929, _G932]. This is not particularly informative for beginners, but it's a list containing exactly three distinct logic variables. Any three-element list is an instance of this. When running in this mode, the N #> 0 constraint ensures termination; otherwise, the recursive clause would find the three-element list, but then continue a futile infinite search for further lists (of negative length, which cannot exist). It takes a bit of getting used to Prolog's execution model to really understand why this is the case.