Quick Sort code:

gtq(X,Y) :- X @> Y.  

quicksort( [],[] ).

quicksort( [X | Tail], Sorted) :-
        split( X, Tail, Small, Big),
        quicksort( Small, SortedSmall),
        quicksort( Big, SortedBig),
        conc( SortedSmall, [X | SortedBig], Sorted).

split( _, [], [], []).

split( X,[Y | Tail], [Y | Small], Big) :-  gtq( X, Y),!, split( X, Tail, Small, Big).

split( X, [Y | Tail], Small, [Y | Big] ) :-  split( X, Tail, Small, Big).

conc([],L,L).

conc( [X | L1], L2, [X | L3]) :-  conc( L1, L2, L3).