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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 _mixed_join(iterable, sentinel):
 """concatenate any string type in an intelligent way."""

    iterator = iter(iterable)
    first_item = next(iterator, sentinel)
    if isinstance(first_item, bytes):
        return first_item + b''.join(iterator)
    return first_item + u''.join(iterator)