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@@ -32,12 +32,48 @@ How would you implement such a list comparison?
 First, read this: [[Implementing trees]]
 
 <OL start=3>
-<LI>blah
+<LI>Write an implementation of leaf-labeled trees. You can do something v3-like, 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. 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.)
+
+<LI>The **fringe** of a leaf-labeled tree is the list of values at its leaves,
+ordered from left to right. For example, the fringe of this tree:
+
+		.
+	   / \
+	  .   3
+	 / \
+	1   2
+
+is [1;2;3]. And that is also the fringe of this tree:
+
+		.
+	   / \
+	  1   .
+	     / \
+        2   3
+
+The two trees are different, but they have the same fringe. We're going to
+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.)
+
+Write the fringe-enumeration 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.)
 </OL>
 
 
+
 #Mutually-recursive functions#
 
 <OL start=4>