code: def sjoiner(inbox, s=None, join=""):
      
      return join.join([input_ for i, input_ in enumerate(inbox) if i in s])
docs: 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()``.