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<?php 
/* Turn on menu descriptions */
function prefix_nav_description( $item_output, $item, $depth, $args ) {
    if ( !empty( $item->description ) ) {
        $item_output = str_replace( $args->link_after . '</a>', '<span class="menu-item-description">' . $item->description . '</span>' . $args->link_after . '</a>', $item_output );
    }
 
    return $item_output;
}
add_filter( 'walker_nav_menu_start_el', 'prefix_nav_description', 10, 4 );
?>