code: def relevant_part(self, original, pos):
          
          start = original.rfind(self._separator, 0, pos)
          if start == -1:
              start = 0
          else:
              start = start + len(self._separator)
          end = original.find(self._separator, pos - 1)
          if end == -1:
              end = len(original)
          return original[start:end], start, end, pos - start
docs: calculates the subword of `original` that `pos` is in