def __sub__(self, other):
        return self.database.sdiff(self.key, other.key)
exception in trait contribution
dataset/ETHPy150Open coleifer/walrus/walrus/containers.py/Set.__sub__--crossover with--dataset/ETHPy150Open enthought/envisage/envisage/tests/plugin_test_case.py/PluginTestCase.test_exception_in_trait_contribution