public function register(
        $doctype,
        $xml = true,
        $modules = array(),
        $tidy_modules = array(),
        $aliases = array(),
        $dtd_public = null,
        $dtd_system = null
    ) {
        if (!is_array($modules)) {
            $modules = array($modules);
        }
        if (!is_array($tidy_modules)) {
            $tidy_modules = array($tidy_modules);
        }
        if (!is_array($aliases)) {
            $aliases = array($aliases);
        }
        if (!is_object($doctype)) {
            $doctype = new HTMLPurifier_Doctype(
                $doctype,
                $xml,
                $modules,
                $tidy_modules,
                $aliases,
                $dtd_public,
                $dtd_system
            );
        }
        $this->doctypes[$doctype->name] = $doctype;
        $name = $doctype->name;
        // hookup aliases
        foreach ($doctype->aliases as $alias) {
            if (isset($this->doctypes[$alias])) {
                continue;
            }
            $this->aliases[$alias] = $name;
        }
        // remove old aliases
        if (isset($this->aliases[$name])) {
            unset($this->aliases[$name]);
        }
        return $doctype;
    }