<OL start=3>
<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.
-
-<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:
+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.
+
+<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:
.
/ \