MrClay\Cli::addArgument PHP Method

    public function addArgument($letter, $required, Arg $arg = null)
    {
        if (!preg_match('/^[a-zA-Z]$/', $letter)) {
            throw new InvalidArgumentException('$letter must be in [a-zA-Z]');
        }
        if (!$arg) {
            $arg = new Arg($required);
        }
        $this->_args[$letter] = $arg;
        return $arg;
    }