
diff git a/week4.mdwn b/week4.mdwn
index e69832fd..5e219484 100644
 a/week4.mdwn
+++ b/week4.mdwn
@@ 6,11 +6,11 @@ A: That's easy: let `T` be an arbitrary term in the lambda calculus. If
`T` has a fixed point, then there exists some `X` such that `X <~~>
TX` (that's what it means to *have* a fixed point).
let W = \x.T(xx) in
let X = WW in
X = WW = (\x.T(xx))W = T(WW) = TX

+
+let L = \x. T (x x) in
+let X = L L in
+X ≡ L L ≡ (\x. T (x x)) L ~~> T (L L) ≡ T X
+
Please slow down and make sure that you understand what justified each
of the equalities in the last line.