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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def derived_from(self, identity: QualName) -> MutableSet[QualName]: """Return list of identities transitively derived from `identity`."""

        try:
            res = self.identity_adjs[identity].derivs
        except KeyError:
            return set()
        for id in res.copy():
            res |= self.derived_from(id)
        return res