myappend([], Y, Y).
myappend([H|X], Y, [H|Z]) :- myappend(X, Y, Z).

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

% try
% myappend([1,2,3], [4,5], Zs).
% myappend([1,2,3], Ys, [1,2,3,4]).
% myappend(Xs, Ys, [1,2,3]).