[Twisted-Python] Waiting for multiple events

[Andrew Bennetts]

I have a collection of Deferreds.  When they all complete, I want to trigger
a "all complete" callback (probably via a Deferred).  Alternatively, upon an
error from any of the callbacks, I want to trigger the errback of that
"all complete" Deferred.

Is there something already written to do this?  Or do I need to write it
myself?

-Andrew.