%------------------------------------------------------
% less efficient
%------------------------------------------------------

ev(0) .
ev(N) :- M is N-1, M > 0, od(M).
                                                                    
od(N) :- M is N-1, ev(M).

tev(N) :- time(ev(N)).
tod(N) :- time(od(N)).


%------------------------------------------------------
% more efficient
%------------------------------------------------------

even(0).
even(X) :- M is floor(X/2)*2, M = X.

odd(X) :- not(even(X)).

teven(N) :- time(even(N)).
todd(N) :- time(odd(N)).