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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 namespace(self, mid: ModuleId) -> YangIdentifier: """Return the namespace corresponding to a module or submodule. Args: mid: Module identifier. Raises: ModuleNotRegistered: If `mid` is not registered in the data model. """

        try:
            mdata = self.modules[mid]
        except KeyError:
            raise ModuleNotRegistered(*mid) from None
        return mdata.main_module[0]