def put(self, x):
if self._num_serial > <NUM_LIT:0> or len(self._threads) == <NUM_LIT:0>:<EOL><INDENT>self._num_serial -= <NUM_LIT:1><EOL>out = self._parallizable.forward_backward(x)<EOL>self._out_queue.put(out)<EOL><DEDENT>else:<EOL><INDENT>self._in_queue.put(x)<EOL><DEDENT>
Assign input `x` to an available worker and invoke
        `parallizable.forward_backward` with x.