n(N1) :-
   N1 > 0,
   N2 is N1-1,
   n(N2).
n(0).

m(0).
m(N1) :-
   N1 > 0,
   N2 is N1-1,
   m(N2).

ten.
ten.
ten.
ten.
ten.
ten.
ten.
ten.
ten.
ten.

append([], Xs, Xs).
append([X|Xs], Ys, [X|Zs]) :-
   append(Xs, Ys, Zs).

inf :-
   inf.


% length(L, N) :-
%   (  var(N) -> length(L, 0, N)
%   ;  must_be(not_less_than_zero, N),