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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 _execute_callback(async, callback):
 """Execute the given callback or insert the Async callback, or if no callback is given return the async.result. """

    from furious.async import Async

    if not callback:
        return async.result.payload

    if isinstance(callback, Async):
        return callback.start()

    return callback()