diff --git a/fourcell/normalize.py b/fourcell/normalize.py
index b4a95b1..5b168fc 100644
--- a/fourcell/normalize.py
+++ b/fourcell/normalize.py
@@ -61,6 +61,8 @@ def is_close (point, points) :
 			return True
 	return False
 
+# taken from
+# https://gist.github.com/phn/1111712/35e8883de01916f64f7f97da9434622000ac0390
 def normalize_angle (num, lower=0.0, upper=360.0, b=False):
     """Normalize number to range [lower, upper) or [lower, upper].
     Parameters