def add_extension(self, extension, critical):
        """
        Adds an X.509 extension to the certificate revocation list.
        """
        if not isinstance(extension, ExtensionType):
            raise TypeError("extension must be an ExtensionType")

        extension = Extension(extension.oid, critical, extension)
        _reject_duplicate_extension(extension, self._extensions)
        return CertificateRevocationListBuilder(
            self._issuer_name, self._last_update, self._next_update,
            self._extensions + [extension], self._revoked_certificates
        )

        Adds an X.509 extension to the certificate revocation list.
        
stackoverflow/venv/lib/python3.6/site-packages/cryptography/x509/base.py