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

        $iconClasses = $icon . ' fa-stack-1x';

        if (count($classes) > 0) {
            foreach ($classes as $class) {
                $iconClasses .= ' ' . $this->classMapper($class);
            }
        }

        $this->bottomIcon = sprintf(self::ICON_HTML, $style, $iconClasses);

        return $this;
    }