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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 get_asset_temporal_assignment_session_for_repository(self, repository_id, proxy):
 """Gets the session for assigning temporal coverage of an asset for the given repository. arg: repository_id (osid.id.Id):
 the Id of the repository arg proxy (osid.proxy.Proxy):
 a proxy return: (osid.repository.AssetTemporalAssignmentSession) - an AssetTemporalAssignmentSession raise: NotFound - repository_id not found raise: NullArgument - repository_id is null raise: OperationFailed - unable to complete request raise: Unimplemented - supports_asset_temporal_assignment() or supports_visible_federation() is false compliance: optional - This method must be implemented if supports_asset_temporal_assignment() and supports_visible_federation() are true. """

        if not repository_id:
            raise NullArgument()
        if not self.supports_asset_temporal_assignment():
            raise Unimplemented()
        try:
            from . import sessions
        except ImportError:
            raise OperationFailed('import error')
        proxy = self._convert_proxy(proxy)
        try:
            session = sessions.AssetTemporalAssignmentSession(repository_id, proxy, runtime=self._runtime)
        except AttributeError:
            raise OperationFailed('attribute error')
        return session