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()``.

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])