Where is Bootstrap walker file in Sage 9.0.0 beta 1

sage
1

I am looking to implement a navigation menu using the Bootstrap4 navigation styling. I have the navigation being outputted but I want to edit the walker class to fit my current Bootstrap4 markup but I can’t find where the walker class file lives?

I created a file in the

/src

directory called

navigation.php

which contains this code

<?php
    namespace App;
    // Add to functions, or to a dedicated controllers file
    // Needs a global body class to hook into
    /**
     * Navigation arguments
     *
     * @param $data
     *
     * @return mixed
     */
    function navControl( $data ) {
        // Pass the walker class to a var, so it
        // doesn't instantiate here.
        $bootstrapWalker = 'wp_bootstrap_navwalker';
        // Main Nav
        $mainNavArgs = [
            'theme_location' => 'primary_navigation',
            'walker'         => new $bootstrapWalker,
            'menu_class'     => 'navbar-nav mr-auto'
        ];
        $data['mainNavArgs'] = $mainNavArgs;
        return $data;
    }
    add_filter( 'sage/template/global/data', 'App\\navControl' );

I put this snippet in the helpers.php file

add_filter( 'body_class', function( $classes ) {
    return array_merge( $classes, array( 'global' ) );
} );

and finally in my header.blade.php file I have

@if (has_nav_menu('primary_navigation'))
    {!! wp_nav_menu($mainNavArgs) !!}
@endif

I am wondering where the walker class is being referenced from?