print_list([]) :-!.
print_list(L) :- L=[H|T], write(H),nl, print_list(T).

rprint_list([]) :- !.
rprint_list([H|T]) :- rprint_list(T), write(H),nl.


exist(X,[H|_]) :-  X=H ,!.
exist(X,[_|T]) :-  exist(X,T).

add_to_front(X,L1,L2) :- not(exist(X,L1)), L2=[X|L1],!.
add_to_front(X,L1,L1).

list_length([],0):- !.
list_length([_|T],R) :-  list_length(T,V), R is V+1.


max_element([H|T],R) :- max_element2([H|T],-1,R).
max_element2([],V,V) :- !.
max_element2([H|T],V,R) :- H > V,  max_element2(T,H,R), !.
max_element2([_|T],V,R) :-  max_element2(T,V,R).