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

Adding my answer to this old thread:

notmember(_,[]).
notmember(X,[H|T]):-X\=H,notmember(X,T).

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

The only virtue of this solution is that it uses only those predicates that have been introduced by the point where this exercise appears in the original text.