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diff --git a/assignment3.mdwn b/assignment3.mdwn
index 289f818a..e240b732 100644
--- a/assignment3.mdwn
+++ b/assignment3.mdwn
@@ -1,6 +1,19 @@
Assignment 3
------------
+Erratum corrected 11PM Sun 3 Oct: the following line
+
+ let tb = (make_list t12 (make_list t3 empty)) in
+
+originally read
+
+ let tb = (make_list t12 t3) in
+
+This has been corrected below, and in the preloaded evaluator for
+working on assignment 3, available here: [[assignment 3 evaluator]].
+
+
+
Once again, the lambda evaluator will make working through this
assignment much faster and more secure.
@@ -29,6 +42,7 @@ Recall that version 1 style lists are constructed like this (see
; church numerals
let iszero = \n. n (\x. false) true in
let succ = \n s z. s (n s z) in
+ let add = \l r. l succ r in
let mul = \m n s. m (n s) in
let pred = (\shift n. n shift (make\_pair 0 0) get\_snd) (\p. p (\x y. make\_pair (succ x) x)) in
let leq = \m n. iszero(n pred m) in
@@ -37,6 +51,7 @@ Recall that version 1 style lists are constructed like this (see
; a fixed-point combinator for defining recursive functions
let Y = \f. (\h. f (h h)) (\h. f (h h)) in
let length = Y (\length l. isempty l 0 (succ (length (tail l)))) in
+ let fold = Y (\f l g z. isempty l z (g (head l)(f (tail l) g z))) in
eq 2 2 yes no
@@ -117,7 +132,7 @@ Expected behavior:
let t12 = (make_list t1 (make_list t2 empty)) in
let t23 = (make_list t2 (make_list t3 empty)) in
let ta = (make_list t1 t23) in
- let tb = (make_list t12 t3) in
+ let tb = (make_list t12 (make_list t3 empty)) in
let tc = (make_list t1 (make_list t23 empty)) in
sum-leaves t1 ~~> 1