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%uni(+SETA,+SETB,-SETC).
%method1
%uni(A,[],A).
%uni(A,[H|T],[H|B]):-uni(A,T,B),not(member(H,A)).
%uni(A,[H|T],B):-uni(A,T,B), member(H,A).

%method2
%uni(A,B,X):-union(A,B,X).

uni(A,[],A).
uni(A,[HB|TB],[HB|TC]):-uni(A,TB,TC),not(member(HB,A)).
uni(A,[HB|TB],TC):-uni(A,TB,TC), member(HB,A).