'Kill off worker threads (not below self.min).'

def shrink(self, amount):

    for t in self._threads:
        if (not t.isAlive()):
            self._threads.remove(t)
            amount -= 1
    if (amount > 0):
        for i in xrange(min(amount, (len(self._threads) - self.min))):
            self._queue.put(_SHUTDOWNREQUEST)
