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diff --git a/TODO.md b/TODO.md
index 1eb6b92..3d7836a 100644
--- a/TODO.md
+++ b/TODO.md
@@ -385,3 +385,4 @@ this stuff may not make any sense.
   (led toggling, wifi)
   (looking at building outdoor sensor and submitting to weather.gov
   cwop, see <https://madis.noaa.gov/madis_cwop.shtml>)
+- combinations and permutations article (partial)