add demo docs

demo/kitchen-sink/docs/prolog.plg

@@ -0,0 +1,18 @@
+partition([], _, [], []).
+partition([X|Xs], Pivot, Smalls, Bigs) :-
+    (   X @< Pivot ->
+        Smalls = [X|Rest],
+        partition(Xs, Pivot, Rest, Bigs)
+    ;   Bigs = [X|Rest],
+        partition(Xs, Pivot, Smalls, Rest)
+    ).
+ 
+quicksort([])     --> [].
+quicksort([X|Xs]) -->
+    { partition(Xs, X, Smaller, Bigger) },
+    quicksort(Smaller), [X], quicksort(Bigger).
+
+perfect(N) :-
+    between(1, inf, N), U is N // 2,
+    findall(D, (between(1,U,D), N mod D =:= 0), Ds),
+    sumlist(Ds, N).