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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 get_tracer( app: Application, tracer_key: str = APP_AIOZIPKIN_KEY) -> Tracer: """Returns tracer object from application context. By default tracer has APP_AIOZIPKIN_KEY in aiohttp application context, you can provide own key, if for some reason default one is not suitable. """

    return cast(Tracer, app[tracer_key])