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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 flatten(*caches):
 """Flatten a nested list of cache entries Parameters *caches : `list` One or more lists of file paths (`str` or :class:`~lal.utils.CacheEntry`). Returns ------- flat : `list` A flat `list` containing the unique set of entries across each input. """

    return list(OrderedDict.fromkeys(e for c in caches for e in c))