function setIssuerException(/*callable*/ $callable)
    {
        if (!is_callable($callable))
            throw new \InvalidArgumentException(sprintf(
                'Issuer must be callable; given: (%s).'
                , \Poirot\Std\flatten($callable)
            ));
        
        $this->issuer_exception = $callable;
        return $this;
    }