protected function setMask($icon, $style='fas')
    {
        if (is_string($icon) === false) {
            throw new \InvalidArgumentException(
                'The mask icon label must be a string.'
            );
        }
        if (is_string($style) === false) {
            throw new \InvalidArgumentException(
                'The mask style label must be a string.'
            );
        }
        if (!in_array($style, $this->STYLES)) {
            throw new \InvalidArgumentException(
                'Invalid mask style.'
            );
        }

        $this->mask = $style." fa-".$icon;

        return $this;
    }