'Testing difference().'

def test_difference(self):

    for i in xrange(len(self.geometries.topology_geoms)):
        a = fromstr(self.geometries.topology_geoms[i].wkt_a)
        b = fromstr(self.geometries.topology_geoms[i].wkt_b)
        d1 = fromstr(self.geometries.diff_geoms[i].wkt)
        d2 = a.difference(b)
        self.assertEqual(d1, d2)
        self.assertEqual(d1, (a - b))
        a -= b
        self.assertEqual(d1, a)
