public function build($interchange, $hash)
    {
        if (!$hash instanceof HTMLPurifier_StringHash) {
            $hash = new HTMLPurifier_StringHash($hash);
        }
        if (!isset($hash['ID'])) {
            throw new HTMLPurifier_ConfigSchema_Exception('Hash does not have any ID');
        }
        if (strpos($hash['ID'], '.') === false) {
            if (count($hash) == 2 && isset($hash['DESCRIPTION'])) {
                $hash->offsetGet('DESCRIPTION'); // prevent complaining
            } else {
                throw new HTMLPurifier_ConfigSchema_Exception('All directives must have a namespace');
            }
        } else {
            $this->buildDirective($interchange, $hash);
        }
        $this->_findUnused($hash);
    }