code: def _setup_index(index):
      
      index = int(index)
      if index > 0:
          index -= 1
      elif index == 0:
          # Zero indicies should not be allowed by default.
          raise ValueError
      return index
docs: Shifts indicies as needed to account for one based indexing
  
      Positive indicies need to be reduced by one to match with zero based
      indexing.
  
      Zero is not a valid input, and as such will throw a value error.
  
      Arguments:
          index -     index to shift