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.
+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: