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(Xs, Ys, [1,2,3]).
 ;
 ;
 ;
 ;
 */