diff --git a/Lib/datetime.py b/Lib/datetime.py
index b9719cb..2f94218 100644
--- a/Lib/datetime.py
+++ b/Lib/datetime.py
@@ -896,6 +896,7 @@
 
         ISO calendar algorithm taken from
         http://www.phys.uu.nl/~vgent/calendar/isocalendar.htm
+        (used with permission)
         """
         year = self._year
         week1monday = _isoweek1monday(year)