WordPress/wp-includes/category.php

<?php
/**
 * WordPress Category API
 *
 * @package WordPress
 */

/** Core category functionality */
require_once( ABSPATH . WPINC . '/category-functions.php' );

/** Walker_Category class */
require_once( ABSPATH . WPINC . '/class-walker-category.php' );

/** Walker_CategoryDropdown class */
require_once( ABSPATH . WPINC . '/class-walker-category-dropdown.php' );