'Adds an X.509 extension to the certificate revocation list.'

def add_extension(self, extension, critical):

    if (not isinstance(extension, ExtensionType)):
        raise TypeError('extension    must    be    an    ExtensionType')
    extension = Extension(extension.oid, critical, extension)
    for e in self._extensions:
        if (e.oid == extension.oid):
            raise ValueError('This    extension    has    already    been    set.')
    return CertificateRevocationListBuilder(self._issuer_name, self._last_update, self._next_update, (self._extensions + [extension]), self._revoked_certificates)
