code: def relevant_part(self, original, pos, sep=' '):
          
          start = original.rfind(sep, 0, pos) + 1
          end = original.find(sep, pos - 1)
          if end == -1:
              end = len(original)
          return original[start:end], start, end, pos - start
docs: calculates the subword in a `sep`-splitted list of substrings of
          `original` that `pos` is ia.n