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diff --git a/week9.mdwn b/week9.mdwn
index 913e4bf7..2e80250e 100644
--- a/week9.mdwn
+++ b/week9.mdwn
@@ -100,7 +100,13 @@ Scheme is similar. There are various sorts of reference cells available in Schem
 		(set-box! ycell 3)
 		(+ x (unbox ycell)))
 
-(C has explicit-style mutable variables, too, which it calls *pointers*. But simple variables in C are already mutable, in the implicit style.)
+C has explicit-style mutable variables, too, which it calls *pointers*. But simple variables in C are already mutable, in the implicit style. Scheme also has both styles of mutation. In addition to the explicit boxes, Scheme also lets you mutate unboxed variables:
+
+	(begin
+		(define x 1)
+		(set! x 2)
+		x)
+	; evaluates to 2
 
 When dealing with explicit-style mutation, there's a difference between the types and values of `ycell` and `!ycell` (or in Scheme, `(unbox ycell)`). The former has the type `int ref`: the variable `ycell` is assigned a reference cell that contains an `int`. The latter has the type `int`, and has whatever value is now stored in the relevant reference cell. In an implicit-style framework though, we only have the resources to refer to the contents of the relevant reference cell. `y` in fragment [G] or the C snippet above has the type `int`, and only ever evaluates to `int` values.