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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 close(self):
 """Attempt to gracefully close all connections in the pool. Wait until all pool connections are released, close them and shut down the pool. If any error (including cancellation) occurs in ``close()`` the pool will terminate by calling :meth:`Pool.terminate() <pool.Pool.terminate>`. It is advisable to use :func:`python:asyncio.wait_for` to set a timeout. .. versionchanged:: 0.16.0 ``close()`` now waits until all pool connections are released before closing them and the pool. Errors raised in ``close()`` will cause immediate pool termination. """

        if self._closed:
            return
        self._check_init()

        self._closing = True

        warning_callback = None
        try:
            warning_callback = self._loop.call_later(
                60, self._warn_on_long_close)

            release_coros = [
                ch.wait_until_released() for ch in self._holders]
            await asyncio.gather(*release_coros, loop=self._loop)

            close_coros = [
                ch.close() for ch in self._holders]
            await asyncio.gather(*close_coros, loop=self._loop)

        except Exception:
            self.terminate()
            raise

        finally:
            if warning_callback is not None:
                warning_callback.cancel()
            self._closed = True
            self._closing = False