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 PaternMatchingHaskellStyle {
   public append(LinkedElem(a, Elem(e)), l2) {
   	  return new LinkedElem<>(a, new LinkedElem<>(e, l2));
   }

   public append(LinkedElem(a, r), l2) {
   	  return new LinkedElem<>(a, append(r, l2));
   }
}