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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(l):
 """helper to flatten a list of lists """

    res = []
    for sublist in l:
        if isinstance(sublist, whaaaaat.Separator):
            res.append(sublist)
        else:
            for item in sublist:
                res.append(item)
    return res