public function set($fx = [], $junk = null)
    {
        if (!$fx) {
            return $this;
        }

        if ($this->fx) {
            throw new Exception([
                'Callback for this Virtual Page is already defined',
                'vp'     => $this,
                'old_fx' => $this->fx,
                'new_fx' => $fx,
            ]);
        }
        $this->fx = $fx;

        return $this;
    }