def __sub__(self, other):
        if isinstance(other, Distance):
            return Distance(default_unit=self._default_unit, m=(self.m - other.m))
        else:
            raise TypeError('Distance must be subtracted from Distance')
dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.2/django/contrib/gis/measure.py/Distance.__sub__/original