def to_si(self, values, from_unit):
        """Return values in SI and the units to which the values have been converted."""
        if from_unit in self.si_units:
            return values, from_unit
        elif from_unit == 'ton':
            return self.to_unit(values, 'tonne', from_unit), 'tonne'
        else:
            return self.to_unit(values, 'kg', from_unit), 'kg'
Return values in SI and the units to which the values have been converted.