public function start_lvl( &$output, $depth = 0, $args = array() )
	{
		// depth dependent classes
		$indent = ( $depth > 0  ? str_repeat( "\t", $depth ) : '' ); // code indent
		$display_depth = ( $depth + 1); // because it counts the first submenu as 0
		$classes = array(
			'sub-menu',
			( $display_depth == 0 ? 'dropdown' : '' ),
			( $display_depth > 0 ? 'dropdown-menu' : '' ),
		);
		$class_names = implode( ' ', $classes );

		// build html
		$output .= "\n" . $indent . '<ul class="' . $class_names . '">' . "\n";
	}