'Compare two UnitDbl\'s.
= ERROR CONDITIONS
- If the input rhs units are not the same as our units,
an error is thrown.
= INPUT VARIABLES
- rhs    The UnitDbl to compare against.
= RETURN VALUE
- Returns -1 if self < rhs, 0 if self == rhs, +1 if self > rhs.'

def __cmp__(self, rhs):

    self.checkSameUnits(rhs, u'compare')
    return cmp(self._value, rhs._value)
