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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 set_preprocessor(self, message_type, preprocessor, executor):
        '''
        Sets PREPROCESSOR to run on MESSAGE_TYPE in EXECUTOR.

        PREPROCESSOR: fn(message_content: bytes) -> PreprocessorResult
        '''
        self._preprocessors[message_type] = \
            _PreprocessorManager(
                executor=executor,
                preprocessor=preprocessor)