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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 forget(request):
 """Called to forget the userid for a request Args: request: aiohttp Request object Raises: RuntimeError: Middleware is not installed """

    auth_policy = request.get(POLICY_KEY)
    if auth_policy is None:
        raise RuntimeError('auth_middleware not installed')

    return await auth_policy.forget(request)