-4. (Still easy) Now write the same function, but don't use the length function (hint: use `leq` as a model).
-
-5. In assignment 2, we discovered that version 3-type lists (the ones that
-work like Church numerals) made it much easier to define operations
-like map and filter. But now that we have recursion in our toolbox,
-reasonable map and filter functions for version 3 lists are within our
-reach. Give definitions for such a map and a filter.
-
-6. Linguists analyze natural language expressions into trees.
-We'll need trees in future weeks, and tree structures provide good
-opportunities for learning how to write recursive functions.
-Making use of the resources we have at the moment, we can approximate
-trees as follows: instead of words, we'll use Church numerals.
-Then a tree will be a (version 1 type) list in which each element is
-itself a tree. For simplicity, we'll adopt the convention that
-a tree of length 1 must contain a number as its only element.
-Then we have the following representations:
-
-<pre>
- (a) (b) (c)
- .
- /|\ /\ /\
- / | \ /\ 3 1/\
- 1 2 3 1 2 2 3
-
-[[1];[2];[3]] [[[1];[2]];[3]] [[1];[[2];[3]]]
-</pre>
-
-Limitations of this scheme include the following: there is no easy way
-to label a constituent (typically a syntactic category, S or NP or VP,
-etc.), and there is no way to represent a tree in which a mother has a
-single daughter.
-
-When processing a tree, you can test for whether the tree contains
-only a numeral (in which case the tree is leaf node) by testing for
-whether the length of the list is less than or equal to 1. This will
-be your base case for your recursive functions that operate on trees.
-
-Write a function that sums the number of leaves in a tree.
-Expected behavior:
-
-let t1 = (make-list 1 nil)
-let t2 = (make-list 2 nil)
-let t3 = (make-list 3 nil)
-let t12 = (make-list t1 (make-list t2 nil))
-let t23 = (make-list t2 (make-list t3 nil))
-let ta = (make-list t1 t23)
-let tb = (make-list t12 t3)
-let tc = (make-list t1 (make-list t23 nil))