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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:
 async def wait_closed(self):
 """Coroutine to wait until all existing request handlers will exit properly. """

        if self._server is None:
            raise RuntimeError('Server is not started')
        await self._server.wait_closed()
        if self._handlers:
            await asyncio.wait({h.wait_closed() for h in self._handlers},
                               loop=self._loop)