myappend([], Ys, Ys).
myappend([H|Xs], Ys, [H|Zs]) :- myappend(Xs, Ys, Zs).

myconcat([], []).
myconcat([Xs|Xss], Ys) :- myappend(Xs, Zs, Ys), myconcat(Xss, Zs).