def _parse_value(cls, stream_rdr, offset, value_count, value_offset):
        """
        Return the rational (numerator / denominator) value at *value_offset*
        in *stream_rdr* as a floating-point number. Only supports single
        values at present.
        """
        if value_count == 1:
            numerator = stream_rdr.read_long(value_offset)
            denominator = stream_rdr.read_long(value_offset, 4)
            return numerator / denominator
        else:  # pragma: no cover
            return 'Multi-value Rational NOT IMPLEMENTED'