public function addAcceptable(string $acceptable, float $priority = 1.0): self
    {
        if (0 > $priority || 1.0 < $priority) {
            throw new \InvalidArgumentException(
                    'Invalid priority, must be between 0 and 1.0'
            );
        }

        $this->acceptables[$acceptable] = $priority;
        return $this;
    }