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diff --git a/week9.mdwn b/week9.mdwn
index 7028e892..568fc812 100644
--- a/week9.mdwn
+++ b/week9.mdwn
@@ -304,6 +304,15 @@ For expressions we already know how to interpret, `s'` will usually just be `s`.
 			eval expr2 ((c, value) :: g) s'
 		...
 
+Similarly:
+
+		...
+		| Addition (expr1, expr2) ->
+			let (value1, s') = eval expr1 g s
+			in let (value2, s'') = eval expr2 g s'
+			in (value1 + value2, s'')
+		...
+
 Let's consider how to interpet our new syntactic forms `newref`, `deref`, and `setref`: