code: def _create_ca_file(anchor_list, filename):
      
      try:
          with open(filename, "w") as f:
              for a in anchor_list:
                  s = a.output(fmt="PEM")
                  f.write(s)
      except IOError:
          return None
      return filename
docs: Concatenate all the certificates (PEM format for the export) in
      'anchor_list' and write the result to file 'filename'. On success
      'filename' is returned, None otherwise.
  
      If you are used to OpenSSL tools, this function builds a CAfile
      that can be used for certificate and CRL check.