diff --git a/py/brown/brown.py b/py/brown/brown.py
index f2bec87..69b031a 100644
--- a/py/brown/brown.py
+++ b/py/brown/brown.py
@@ -40,8 +40,10 @@ class BrownIterator(object):
 		self.px, self.py = self._interaction(self.x, self.y, self.px, self.py)
 		# XXX: We need the (-1)**i to make the problem
 		# symmetric.
+		# FIXME: is this necessary?
 		self.px[np.isnan(self.px)] = self.speed_of_light * (-1)**self._i
 		self.py[np.isnan(self.py)] = self.speed_of_light * (-1)**self._i
+
 		self._reflect_at_borders()
 
 		self.x += self.dt * self.px