Liste

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

tail([X],[]).
tail([X|Y],Y).
#tail([_],[]).
#tail([_|Y],Y).

element_of(E,[E|_]).
element_of(E,[X|Y]):-E\=X,element_of(E,Y).

len([],0).
len([_|Y],N):-len(Y|N1),N is 1+N1.

concat([],Y,Y).
concat([Xh|Xt],Y,[Xh|Z]):- concat(Xt,Y,Z).

rev([],[]).
rev([Xh,Xt],Z):- rev(Xt,Y1),concat(Y1,[Xh],Z).

insert([],E,[E]).
insert([Xh|Xt],E,[E,Xh|Xt]):-E=<Xh.
insert([Xh|Xt],E,[Xh|Y]):-E>Xh,insert(Xt,E,Y).