@define listapp_(l1,l2) {
  if (!islist(l2)) {
    append(l1,l2);
    return l1;
  }
  while (!isempty(l2)) {
    switch (l2) {
    @match []: break;
    @match [h,t...]: append(l1,h); l2 = t; break;
    }
  }
  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]]));