% app3(Xs, Ys, Zs, Us) :- Us is the result of concatenating the lists Xs, Ys and Zs
app3(Xs, Ys, Zs, Us) :-	app(Xs, Ys, Vs), app(Vs, Zs, Us).

% augmented by the APPEND program
app([], Ys, Ys).
app([X | Xs], Ys, [X | Zs]) :- app(Xs, Ys, Zs).