+4. Now write the same function, but don't use the length function (hint: use `leq` as a model).
+
+##Trees##
+
+Since we'll be working with linguistic objects, let's approximate
+trees as follows: a tree is a version 1 list
+a Church number is a tree, and
+if A and B are trees, then (make-pair A B) is a tree.
+