Example of unnecessarily calling get_template_directory() over and over again.

functions.php

<?php

// Start the engine.
include_once( get_template_directory() . '/lib/init.php' );

// Setup Theme.
include_once( get_stylesheet_directory() . '/lib/theme-defaults.php' );

// Set Localization (do not remove).
add_action( 'after_setup_theme', 'altitude_localization_setup' );
function altitude_localization_setup(){
	load_child_theme_textdomain( 'altitude-pro', get_stylesheet_directory() . '/languages' );
}

// Add the theme helper functions.
include_once( get_stylesheet_directory() . '/lib/helper-functions.php' );

// Add Image upload and Color select to WordPress Theme Customizer.
require_once( get_stylesheet_directory() . '/lib/customize.php' );

// Include Customizer CSS.
include_once( get_stylesheet_directory() . '/lib/output.php' );

// Include the WooCommerce setup functions.
include_once( get_stylesheet_directory() . '/lib/woocommerce/woocommerce-setup.php' );

// Include the WooCommerce custom CSS if customized.
include_once( get_stylesheet_directory() . '/lib/woocommerce/woocommerce-output.php' );

// Include notice to install Genesis Connect for WooCommerce.
include_once( get_stylesheet_directory() . '/lib/woocommerce/woocommerce-notice.php' );