append([],L,L).
append([H|L1],L2,[H|L3]):-
      append(L1,L2,L3).


member(H,[H|_]).
member(H,[_|T]):-
      member(H,T).


rev(L,Revl):-
      rev(L,[],Revl).

rev([],L,L).
rev([H|L1],L2,M):-
      rev(L1,[H|L2],M).

nexto(X,Y,[X,Y|_]).
nexto(X,Y,[_|Z]):-
      nexto(X,Y,Z).

replace(O,N,[O|T],[N|R]):-
      replace(O,N,T,R).
replace(O,N,[X|T],[X|R]):-
      replace(O,N,T,R).
replace(_,_,[],[]).

delete(A,[A|T],T).
delete(A,[H|T],[H|R]):-
      delete(A,T,R).

delall(X,[X|T],M):-
       delall(X,T,M).
delall(X,[H|T],[H|Y]):-
      delall(X,T,Y).
delall(_,[],[]).

nth(1,[H|T],H).
nth(N,[H|T],El):-
      N1 is N-1,
      nth(N1,T,El).


length(N,L):-
      length(N,0,L).

length(Ans,Ans,[]).
length(N,Acc,[H|T]):-
      NewA is Acc + 1,
      length(N,NewA,T).

last(X,[X]).
last(X,[_|Y]):-
      last(X,Y).