public function rule_bool($a)
    {
        // We don't use PHP inbuilt test - a bit restrictive

        // Changes PHP true/false to 1, 0
        $a = strtolower($a);

        $vals = array('true', 'false', 't', 'f', 1, 0, 'yes', 'no', 'y', 'n');

        if (!in_array($a, $vals)) {
            return $this->fail('Must be a boolean value');
        }
    }