public function getInputBOM(): string
    {
        if (null !== $this->input_bom) {
            return $this->input_bom;
        }

        $this->document->setFlags(SplFileObject::READ_CSV);
        $this->document->rewind();
        $this->input_bom = bom_match((string) $this->document->fread(4));

        return $this->input_bom;
    }