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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 sjoiner(inbox, s=None, join=""):
 """ String joins input with indices in s. Arguments: - s(sequence) [default: ``None``] ``tuple`` or ``list`` of indices of the elements which will be joined. - join(``str``) [default: ``""``] String which will join the elements of the inbox i.e. ``join.join()``. """

    return join.join([input_ for i, input_ in enumerate(inbox) if i in s])