'Add 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 add.
= RETURN VALUE
- Returns the sum of ourselves and the input UnitDbl.'

def __add__(self, rhs):

    self.checkSameUnits(rhs, u'add')
    return UnitDbl((self._value + rhs._value), self._units)
