stack *stack_duplicate(stack *s) {
  stack *s2 = stack_empty();
  if (s2 == NULL) {
    return NULL;
  }
  cstack **pp2 = &(s2 -> head);
  cstack *p = s -> head;
  while (p != NULL) {
    *pp2 = malloc(sizeof **pp2);
    if (*pp2 == NULL) {
      stack_dispose(&s2);
      return NULL;
    }
    (*pp2) -> value = p -> value;
    pp2 = &((*pp2) -> next);
    p = p -> next;
  }
  *pp2 = NULL;
  s2 -> height = s -> height;
  return s2;
}