public function get($doctype)
    {
        if (isset($this->aliases[$doctype])) {
            $doctype = $this->aliases[$doctype];
        }
        if (!isset($this->doctypes[$doctype])) {
            trigger_error('Doctype ' . htmlspecialchars($doctype) . ' does not exist', E_USER_ERROR);
            $anon = new HTMLPurifier_Doctype($doctype);
            return $anon;
        }
        return $this->doctypes[$doctype];
    }