@abc.abstractmethod
  def peek(self, dag):
    pass
Forcing garbage collection should cause unhandled Deferreds to be
dataset/ETHPy150Open ucb-sts/sts/sts/control_flow/peeker.py/Peeker.peek--crossover with--dataset/ETHPy150Open nlloyd/SubliminalCollaborator/libs/twisted/trial/test/test_tests.py/TestUnhandledDeferred.test_isReported