Remove duplicates in list (Prolog)

Question

I am completely new to Prolog and trying some exercises. One of them is:

Write a predicate set(InList,OutList) which takes as input an arbitrary l

Answer 1

You just have to stop the backtracking of Prolog.

enter code here
member(X,[X|_]):- !.
member(X,[_|T]) :- member(X,T).

set([],[]).
set([H|T],[H|Out]) :-
   not(member(H,T)),
   !,
set(T,Out).
set([H|T],Out) :-
   member(H,T),
   set(T,Out).

Answer 2

A simpler (and likely faster) solution is to use library predicate sort/2 which remove duplicates in O(n log n). Definitely works in Yap prolog and SWIPL

Answer 3

This works without cut, but it needs more lines and another argument. If I change the [H2|T2] to S on line three, it will produce multiple results. I don't understand why.

setb([],[],_).
setb([H|T],[H|T2],A) :- not(member(H,A)),setb(T,T2,[H|A]).
setb([H|T],[H2|T2],A) :- member(H,A),setb(T,[H2|T2],A).
setb([H|T],[],A) :- member(H,A),setb(T,[],A).
set(L,S) :- setb(L,S,[]).