diff --git a/propka/bonds.py b/propka/bonds.py
index c0f7e71..e199858 100644
--- a/propka/bonds.py
+++ b/propka/bonds.py
@@ -354,8 +354,9 @@ class bondmaker:
         #print('z range: [%6.2f;%6.2f] %6.2f'%(zmin,zmax,zlen))
 
         # how many boxes do we need in each dimension?
-        # NOTE: math.ceil() returns an int in python3 and a float in python2,
-        # so we need to cast it to int for range() to work.
+        # NOTE: math.ceil() returns an int in python3 and a float in
+        # python2, so we need to convert it to an int for range() to work in
+        # both versions. See PEP 3141.
         self.no_box_x = max(1, int(math.ceil(xlen/box_size)))
         self.no_box_y = max(1, int(math.ceil(ylen/box_size)))
         self.no_box_z = max(1, int(math.ceil(zlen/box_size)))