domains
 s=symbol
 ls=s*

predicates
 elem(s,ls)
 usup(ls,ls)

clauses
 elem(X,[X|_]).
 elem(X,[_|O]):-elem(X,O).
 usup([],[]).
 usup([G|O],L):-elem(G,O),!,usup(O,L).
 usup([G|O],L):-usup(O,L1),L=[G|L1].