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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 _evaluate(self):
 """Lazily retrieve and build User instances from returned data"""

        if self._elements:
            for element in self._elements:
                yield element
        else:
            for user_id in self.__user_ids:
                element = self._swimlane.users.get(id=user_id)
                self._elements.append(element)
                yield element