'Process the headers sent by the server to this client connection.
\'key\' is the websocket handshake challenge/response key.'

def _process_server_headers(self, key, headers):

    assert (headers['Upgrade'].lower() == 'websocket')
    assert (headers['Connection'].lower() == 'upgrade')
    accept = self.compute_accept_value(key)
    assert (headers['Sec-Websocket-Accept'] == accept)
    extensions = self._parse_extensions_header(headers)
    for ext in extensions:
        if ((ext[0] == 'permessage-deflate') and (self._compression_options is not None)):
            self._create_compressors('client', ext[1])
        else:
            raise ValueError('unsupported    extension    %r', ext)
