% ['append.pl'].
% append([apple,grapes],[peach,mango],X).
% append([apple,grapes],[apple,mango],X).
% append([apple,grapes],[],X).
% append(X,Y,[a,b,c]).
% append([apple,grapes],[peach,mango],[apple, grapes, peach, mango]).
% append([apple,grapes],[peach,mango],[apple]).

append([], List, List).
append([Head|List_1],List_2,[Head|List_3]) :-
  append(List_1, List_2, List_3).