def METHOD_NAME(self):
    @pipeline.mutator_stage
    def setkey(key, item):
        item[key] = True
    pl = pipeline.Pipeline([
        iter([{'x': False}, {'a': False}]),
        setkey('x'),
    ])
    self.assertEqual(list(pl.pull()),
                     [{'x': True}, {'a': False, 'x': True}])