code: def append(self, s):
          
          if self._compressor is None:
              return zlib.compress(self.s + s, 9)
          else:
              compressor = self._compressor.copy()
              out = self._out
              out += compressor.compress(s)
              return out + compressor.flush()
docs: Append the bytestring `s` to the compressor state and return the
          final compressed output.