public function setValid($charset, $valid = true)
    {
        $charset = Horde_String::upper($charset);
        $valid = (bool)$valid;

        if (!isset($this->_charsets[$charset]) ||
            ($this->_charsets[$charset] !== $valid)) {
            $this->_charsets[$charset] = $valid;
            $this->notify();
        }
    }