def METHOD_NAME() -> None:
    class SubclassMiddleware(AbstractMiddleware):
        exclude = r"^/123"
        async def __call__(self, scope: "Scope", receive: "Receive", send: "Send") -> None:
            async def _send(message: "Message") -> None:
                if message["type"] == "http.response.start":
                    headers = MutableScopeHeaders(message)
                    headers.add("test", str(123))
                await send(message)
            await self.app(scope, receive, _send)
    @get("/123")
    def first_handler() -> dict:
        return {"hello": "world"}
    @get("/456")
    def second_handler() -> dict:
        return {"hello": "world"}
    @asgi("/mount", is_mount=True)
    async def handler(scope: "Scope", receive: "Receive", send: "Send") -> None:
        response = ASGIResponse(body=b"ok", media_type=MediaType.TEXT)
        await response(scope, receive, send)
    with create_test_client(
        [first_handler, second_handler, handler], middleware=[DefineMiddleware(SubclassMiddleware)]
    ) as client:
        response = client.get("/123")
        assert "test" not in response.headers
        response = client.get("/456")
        assert "test" in response.headers
        response = client.get("/mount/123")
        assert "test" in response.headers