week9 tweak

Signed-off-by: Jim Pryor <profjim@jimpryor.net>

diff --git a/week9.mdwn b/week9.mdwn
index 145833d..f154d2a 100644 (file)
--- a/week9.mdwn
+++ b/week9.mdwn
@@ -236,6 +236,14 @@ The core idea to referential transparency is that when the same value is supplie
 
 Notice that the two invocations of `f 1` yield different results, even though the same value is being supplied as an argument to the same function.
 
+Similarly, functions like these:
+
+       let f cell = !cell;;
+
+       let g cell = cell := !cell + 1; !cell;;
+
+may return different results each time they're invoked, even if they're always supplied one and the same reference cell as argument.
+
 Computer scientists also associate referential transparency with a kind of substitution principle, illustrated here:
 
        let x = 1