public function every($unit = null, $value = null): self
    {
        if (null === $unit || !isset($this->fieldsPosition[$unit])) {
            return $this;
        }

        $value = (1 === (int) $value) ? '*' : '*/' . $value;

        return $this->spliceIntoPosition($this->fieldsPosition[$unit], $value)
                    ->applyMask($unit);
    }