XGitUrl: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=assignment4.mdwn;h=9b7ec2c028622dc92e17dced54403aa6043b8896;hp=8b8a19dc78c1b71ec20e64e6df79d8599f245755;hb=339b62e48b39fadd325d9e0dc903430c4a896870;hpb=72c0c8340fbb4f71beef958e74950db4cd677087
diff git a/assignment4.mdwn b/assignment4.mdwn
index 8b8a19dc..9b7ec2c0 100644
 a/assignment4.mdwn
+++ b/assignment4.mdwn
@@ 34,9 +34,16 @@ First, read this: [[Implementing trees]]
 Write an implementation of leaflabeled trees. You can do something v3like, or use the Y combinator, as you prefer.
 You'll need an operation `make_leaf` that turns a label into a new leaf. You'll need an operation `make_node` that takes two subtrees (perhaps leaves, perhaps other nodes) and joins them into a new tree. You'll need an operation `isleaf` that tells you whether a given tree is a leaf. And an operation `extract_label` that tells you what value is associated with a given leaf.
+You'll need an operation `make_leaf` that turns a label into a new leaf. You'll
+need an operation `make_node` that takes two subtrees (perhaps leaves, perhaps
+other nodes) and joins them into a new tree. You'll need an operation `isleaf`
+that tells you whether a given tree is a leaf. And an operation `extract_label`
+that tells you what value is associated with a given leaf. And an operation
+`extract_left` that tells you what the left subtree is of a tree that isn't a
+leaf. (Presumably, `extract_right` will work similarly.)

 The **fringe** of a leaflabeled tree is the list of values at its leaves, ordered from left to right. For example, the fringe of this tree:
+
 The **fringe** of a leaflabeled tree is the list of values at its leaves,
+ordered from left to right. For example, the fringe of this tree:
.
/ \
@@ 44,7 +51,7 @@ First, read this: [[Implementing trees]]
/ \
1 2
is [1;2;3]. And that is also the fringe of this tree:
+is `[1;2;3]`. And that is also the fringe of this tree:
.
/ \
@@ 57,19 +64,22 @@ return later in the term to the problem of determining when two trees have the
same fringe. For now, one straightforward way to determine this would be:
enumerate the fringe of the first tree. That gives you a list. Enumerate the
fringe of the second tree. That also gives you a list. Then compare the two
lists to see if they're equal. (You just programmed this above.)
+lists to see if they're equal.
Write the fringeenumeration function. It should work on the implementation of
trees you designed in the previous step.
+Write the fringeenumeration function. It should work on the
+implementation of trees you designed in the previous step.
(See [[hints/Assignment 4 hint 3]] if you need some hints.)
+Then combine this with the list comparison function you wrote for question 2,
+to yield a samefringe detector. (To use your list comparison function, you'll
+have to make sure you only use Church numerals as the labels of your leaves,
+though nothing enforces this selfdiscipline.)
#Mutuallyrecursive functions#

+
 (Challenging.) One way to define the function `even` is to have it hand off
part of the work to another function `odd`:
@@ 127,7 +137,7 @@ definitions of `even` and `odd`?
notes](/week3/#index4h2) as a model, construct a pair `Y1` and `Y2` that behave
in the way described.
(See [[hints/Assignment 4 hint 4]] if you need some hints.)
+(See [[hints/Assignment 4 hint 3]] if you need some hints.)