docs: def export(self, filename, fmt="DER"):
          """
          Export certificate in 'fmt' format (DER or PEM) to file 'filename'
          """
          f = open(filename, "wb")
          if fmt == "DER":
              f.write(self.der)
          elif fmt == "PEM":
              f.write(self.pem)
          f.close()