code: def _check_header_lines(self, header_lines):
          
          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)
docs: Check header lines, in particular for starting file "##fileformat"