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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).