def _check_header_lines(self, header_lines):
        """Check header lines, in particular for starting file "##fileformat"
        """
        if not header_lines:
            raise exceptions.InvalidHeaderException(
                "The VCF file did not contain any header lines!"
            )
        first = header_lines[0]
        if first.key != "fileformat":
            raise exceptions.InvalidHeaderException("The VCF file did not start with ##fileformat")
        if first.value not in SUPPORTED_VCF_VERSIONS:
            warnings.warn("Unknown VCF version {}".format(first.value), UnknownVCFVersion)