def relevant_part(self, original, pos):
        """
        calculates the subword of `original` that `pos` is in
        """
        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
calculates the subword of `original` that `pos` is in