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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 search_index_file():
 """Return the default local index file, from the download cache"""

    from metapack import Downloader
    from os import environ

    return environ.get('METAPACK_SEARCH_INDEX',
                       Downloader.get_instance().cache.getsyspath('index.json'))