wp-content/themes/twentythirteen/inc/back-compat.php:21Prevent switching to Twenty Thirteen on old versions of WordPress.
function twentythirteen_switch_theme() { switch_theme( WP_DEFAULT_THEME, WP_DEFAULT_THEME ); unset( $_GET['activated'] ); add_action( 'admin_notices', 'twentythirteen_upgrade_notice' );}Introduced in Twenty Thirteen 1.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-content/themes/twentythirteen/inc/back-compat.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.