append([],L,L).
append([A|X],Y,[A|Z]) :- append(X,Y,Z).

cons(X,Y,[X|Y]).

flatten([H|T], RESULT) :- ( atom(H),
                            flatten(T,Rt),
                            cons(H,Rt,RESULT)
                          ) ;
                          ( flatten(H,Rh),
                            flatten(T,Rt),
                            append(Rh,Rt,RESULT)
                          ) .
flatten([],[]).

skeleton([H|T], RESULT) :- (
                             H==[],
                             skeleton(T,Rt),
                             cons(H,Rt,RESULT)
                           );(
                             atom(H),
                             skeleton(T,RESULT)
                           ) ;
                           ( skeleton(H,Rh),
                             skeleton(T,Rt),
                             cons(Rh,Rt,RESULT)
                           ) .
skeleton([],[]).