quicksort :: (Ord a) => [a] -> [a]
  quicksort [] = []
  quicksort (x:xs) =
    quicksort [y | y<-xs, y<x] ++
    [x] ++
    quicksort [y | y<-xs, y>=x]

  not(P) :- call(P), !, fail.
  not(P).