docs: @Nonnull
    public CollectingJSCodeProvider addFlattenedAt (@Nonnegative final int nIndex, @Nullable final IHasJSCode aProvider)
    {
      if (aProvider != null)
        if (aProvider instanceof CollectingJSCodeProvider)
          m_aList.addAll (nIndex, ((CollectingJSCodeProvider) aProvider).m_aList);
        else
          m_aList.add (nIndex, aProvider);
      return this;
    }