sealed interface List<T> permits LinkedElem, Elem {}

public record LinkedElem<T>(T a, List<T> l) implements List<T> {}
public record Elem<T>(T a) implements List<T> {}

public class SwitchAppend {
   public append(l1, l2) {
       return switch(l1) {
           case LinkedElem(a, Elem(e)) -> new LinkedElem<>(a, new LinkedElem<>(e, l2));
           case LinkedElem(a, r) -> new LinkedElem<>(a, append(r, l2));
           //Alternativ:
	       //case LinkedElem(a, LinkedElem(e, r)) -> new LinkedElem<>(a, append(new LinkedElem(e, r), l2));
           default -> null;
       };
   }
}