'Start the execution engine'

@defer.inlineCallbacks
def start(self):

    assert (not self.running), 'Engine    already    running'
    self.start_time = time()
    (yield self.signals.send_catch_log_deferred(signal=signals.engine_started))
    self.running = True
    self._closewait = defer.Deferred()
    (yield self._closewait)
