public function setSkip(int $skipMode): Indexer {
        if (!in_array($skipMode, [self::SKIP_NONE, self::SKIP_NOT_EXIST, self::SKIP_EXIST, self::SKIP_BINARY_EXIST])) {
            throw new BadMethodCallException('Wrong skip mode');
        }
        $this->skipMode = $skipMode;
        return $this;
    }