Hmm... possibly for the patch to Twisted itself.  I'm not entirely sure when it would be used though, since as soon as it fires/errbacks it should be removed from the pool automatically.  I'm not sure whether there'd be a case where the user would add it to the pool, and then change their mind before it callbacks (they could just not put it into the pool).

Even if it's removed, it the callbacks to remove it can't be un-added.  We could get around it by checking if `_pool` has the deferred before removing it and `calling if_empty_notify`, but `_fired` will still be in the deferred's callback chain, and it's not as neat as I'd like.