'Send the next blocked command.
If there are no more commands in the blocked queue, set up for the next
command to be sent immediately.'

def _unblock(self):

    if (self._blockedQueue == []):
        self._blockedQueue = None
    elif (self._blockedQueue is not None):
        _blockedQueue = self._blockedQueue
        self._blockedQueue = None
        (d, f, a) = _blockedQueue.pop(0)
        d2 = f(*a)
        d2.chainDeferred(d)
        self._blockedQueue.extend(_blockedQueue)
