using momentum changes now works.

py/brown/brown.py

@@ -38,8 +38,8 @@ class BrownIterator(object):
 			return self.x, self.y
 
 		delta_px, delta_py = self._interaction(self.x, self.y)
-		self.px += delta_px
-		self.py += delta_py
+		self.px = delta_px + self.px
+		self.py = delta_py + self.py
 		# XXX: We need the (-1)**i to make the problem
 		# symmetric.
 		self.px[np.isnan(self.px)] = self.speed_of_light * (-1)**self._i