docs: @Nonnull
    public static BootstrapButtonGroup createButtonAsDropDownMenu (@Nonnull final BootstrapButton aButton,
                                                                   @Nonnull final Consumer <? super BootstrapDropdownMenu> aMenuItemProvider)
    {
      final BootstrapButtonGroup aBG = new BootstrapButtonGroup ();
      BootstrapDropdown.makeDropdownToggle (aButton);
      aBG.addChild (aButton);
  
      final BootstrapDropdownMenu aMenu = aBG.addDropDownMenu ();
      aMenuItemProvider.accept (aMenu);
      return aBG;
    }