@define listapp_(l1,l2) {
  @local h,t;
  if (!islist(l2)) {
    append(l1,l2);
    return l1;
  }
  t = l2;
  while (!isempty(t)) {
    h = head(t);
    append(l1,h);
    t = tail(t);
  }
  return l1;
}

printf("%a\n", listapp_([],[1,2,3]));
printf("%a\n", listapp_([1,2,3],[]));
printf("%a\n", listapp_([1,2],[3,4,5]));
printf("%a\n", listapp_([[1,2],[3,4,5]],[[1,2]]));