Arguments have two types

[Oleg Dmitrochenko]

I got the description

    /**
     * Adds a new value to the current group or to the given group.
     *
     * @param  string $name
     * @param  mixed  $value
     * @param  string $groupName
     * @return void
     */
    public function add(string $name, $value, string $groupName = null)

Must be:

    /**
     * Adds a new value to the current group or to the given group.
     *
     * @param  string $name
     * @param  mixed  $value
     * @param  string|null $groupName
     * @return void
     */
    public function add(string $name, $value, string $groupName = null)

https://yadi.sk/i/gQW1sLXPn_mEKQ