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function _xe_deactivate() {
  deactivate_plugins( plugin_basename( __FILE__ ) );
}

function _xe_admin_notice() {
   echo '<div class="updated"><p><strong>XeMizer</strong> requires Advanced Custom Fields Pro v5 or later. So has been <strong>deactivated</strong>.</p></div>';
   if ( isset( $_GET['activate'] ) ) {
        unset( $_GET['activate'] );
    }
}

add_action( 'admin_init', '_xe_deactivate' );
add_action( 'admin_notices', '_xe_admin_notice' );