% removes multiple occurrences of an element, 
% keeps only one occurrence of each element (the last one)
% Example:
%?- remove_duplicates([a,b,c,b,b,a],R).
%R = [c, b, a] ;
%No

remove_duplicates([], []).

remove_duplicates([H| T1], T2) :-
	member(H, T1),!, 
	remove_duplicates(T1, T2).

remove_duplicates([H| T1], [H| T2]) :-
	not(member(H, T1)), % can take this our becasue of CUT 
	remove_duplicates(T1, T2).

% Same predicate with accumulator, keeps first occurrence
% Example:
%?- r_duplicates([a,b,c,b,b,a],R).
%R = [a, b, c] ;
%No

r_duplicates(L, R) :- r_helper(L, [], R).

r_helper([],A,A).
r_helper([H|T],A,R):- member(H,T),!,r_helper(T,A,R).
r_helper([H|T],A,R):- r_helper(T,[H|A],R).