void add(T @NotNull [] items) {
      if (items.length == 0) return;
      if (mySingleItem == null) {
        mySingleItem = items;
        return;
      }
      if (myAllItems == null) {
        T[] elements = mySingleItem;
        myAllItems = ContainerUtil.addAll(new HashSet<>(elements.length), elements);
      }
      ContainerUtil.addAll(myAllItems, items);
    }
void add(T @NotNull [] items)