public function addRole($rrid, $label)
    {
        if (!is_integer($rrid)) {
            throw new \InvalidArgumentException("The relative role identifier must be an integer.");
        }
        if (isset($this->roles[$rrid])) {
            throw new \LogicException(sprintf("This relative role identifier is already used (%d).", $rrid));
        }

        $this->roles[$rrid] = $label;
        return $this;
    }