A helper method for Type subclasses to use for a simple way to truncate
        indx bytes.

def _getIndxChop(self, indx):
        '''
        A helper method for Type subclasses to use for a simple way to truncate
        indx bytes.
        '''
        # cut down an index value to 256 bytes...
        if len(indx) <= 256:
            return indx

        base = indx[:248]
        sufx = xxhash.xxh64(indx).digest()
        return base + sufx