code: def when_built(self):
          
          # XXX note to self: we never do an errback; fix this behavior
          if self.state == 'BUILT':
              return defer.succeed(self)
          return self._when_built.when_fired()
docs: Returns a Deferred that is callback()'d (with this Circuit
          instance) when this circuit hits BUILT.
  
          If it's already BUILT when this is called, you get an
          already-successful Deferred; otherwise, the state must change
          to BUILT.
  
          If the circuit will never hit BUILT (e.g. it is abandoned by
          Tor before it gets to BUILT) you will receive an errback