code: def append(self, cert):
          
  
          if not isinstance(cert, x509.Certificate):
              if not isinstance(cert, byte_cls):
                  raise TypeError(pretty_message(
                      '''
                      cert must be a byte string or an
                      asn1crypto.x509.Certificate object, not %s
                      ''',
                      type_name(cert)
                  ))
              if pem.detect(cert):
                  _, _, cert = pem.unarmor(cert)
              cert = x509.Certificate.load(cert)
  
          if cert.issuer_serial in self._cert_hashes:
              raise DuplicateCertificateError()
  
          self._cert_hashes.add(cert.issuer_serial)
          self._certs.append(cert)
  
          return self
docs: Appends a cert to the path. This should be a cert issued by the last
          cert in the path.
  
          :param cert:
              An asn1crypto.x509.Certificate object
  
          :return:
              The current ValidationPath object, for chaining