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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 add_config_source(self, config_source, position=None):
 """ Add a config source to the current ConfigResolver instance. If position is not set, this source will be inserted with the lowest priority. """

        rank = position if position is not None else len(self._config_sources)
        self._config_sources.insert(rank, config_source)