def when_built(self):
        d = defer.Deferred()
        if self.state == 'BUILT':
            d.callback(self)
        else:
            self._when_built.append(d)
        return d
Returns a Deferred that is callback()'d (with this Circuit
dataset/ETHPy150Open meejah/txtorcon/txtorcon/circuit.py/Circuit.when_built