Update coding_principles.md

lecture/coding_principles.md

@@ -122,4 +122,12 @@ have to remember to change the others, or, like the alien computers, your progra
 to its knees by a contradiction. It isn't a question of whether you'll remember: it's a question of when
 you'll forget.
 
-(The Progmatic Programmer: Tip 11)
+(The Progmatic Programmer: Tip 11)
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+
+## 30. asserts
+If It Can't Happen, Use Assertions to Ensure That It Won't 
+
+Don't use assertions in place of real error handling. Assertions check for things that should never
+happen.
+
+(The Progmatic Programmer: Tip 33)
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