How you can embody second layer sub menu to Walker Nav?

I would like to have the ability to entry the sub-menus of the sub-menus on my nav. At the moment I am solely in a position to entry the primary sub-menu layer. What do I would like so as to add to entry these?

<?php


class Walker_Nav_Primary extends Walker_Nav_Menu {

    operate start_lvl( &$output, $depth = 0, $args = array() )  // ul
    {
        $indent = str_repeat("t", $depth);
        $submenu = ($depth > 0) ? 'sub-menu' : '';
        $output .="n$indent<ul class="submenu-first $submenu depth_$depth">n";
    }

  
    operate start_el( &$output, $merchandise, $depth = 0, $args = array(), $id = 0 ){ //li a span
        
        $indent = ( $depth ) ? str_repeat("t",$depth) : '';
        
        $li_attributes = '';
        $class_names = $worth = '';
        
        $lessons = empty( $item->lessons ) ? array() : (array) $item->lessons;
        
        $lessons[] = ($args->walker->has_children) ? 'has_sub' : '';
        $lessons[] = ($item->present || $item->current_item_ancestor) ? 'energetic' : '';
        $lessons[] = 'menu-item-' . $item->ID;
        if( $depth && $args->walker->has_children ){
            $lessons[] = 'dropdown-submenu';
        }

        $class_names =  be part of(' ', apply_filters('nav_menu_css_class', array_filter( $lessons ), $merchandise, $args ) );
        $class_names = ' class="' . esc_attr($class_names) . '"';

        $id = apply_filters('nav_menu_item_id', 'menu-item-'.$item->ID, $merchandise, $args);
        $id = strlen( $id ) ? ' id="' . esc_attr($id) . '"' : "";
        $output .= $indent . '<li' . $id . $worth . $class_names . $li_attributes . '>';

        $attributes = ! empty( $item->attr_title ) ? ' title="' . esc_attr($item->attr_title) . '"' : '';
        $attributes .= ! empty( $item->goal ) ? ' goal="' . esc_attr($item->target) . '"' : '';
        $attributes .= ! empty( $item->xfn ) ? ' rel="' . esc_attr($item->xfn) . '"' : '';
        $attributes .= ! empty( $item->url ) ? ' href="' . esc_attr($item->url) . '"' : '';
        
        $attributes .= ( $args->walker->has_children ) ? ' class="has_sub_link" data-toggle="has_sub_link"' : '';
        
        $item_output = $args->earlier than;
        $item_output .= '<a' . $attributes . '>';
        $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
        $item_output .= ( $depth == 0 && $args->walker->has_children ) ? ' <b class="caret"></b></a>' : '</a>';
        $item_output .= $args->after;
        
        $output .= apply_filters ( 'walker_nav_menu_start_el', $item_output, $merchandise, $depth, $args );
    }
}