<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.
+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:
+<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:
.
/ \
/ \
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:
.
/ \
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>
+<OL start=5>
<LI>(Challenging.) One way to define the function `even` is to have it hand off
part of the work to another function `odd`: