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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def clear_synchronous_cb(self, cb):
        '''
        If the callback is the current expected callback, will clear it off the
        stack.  Else will raise in exception if there's an expectation but this
        doesn't satisfy it.
        '''
        if len(self._pending_events):
            ev = self._pending_events[0]

            # We can't have a strict check using this simple mechanism,
            # because we could be waiting for a synch response while messages
            # are being published. So for now, if it's not in the list, do a
            # check to see if the callback is in the pending list, and if so,
            # then raise, because it means we received stuff out of order.
            # Else just pass it through. Note that this situation could happen
            # on any broker-initiated message.
            if ev == cb:
                self._pending_events.popleft()
                self._flush_pending_events()
                return ev

            elif cb in self._pending_events:
                raise ChannelError(
                    "Expected synchronous callback %s, got %s", ev, cb)
        # Return the passed-in callback by default
        return cb