Guided Practice 6.3: Using tree-fold

Using tree-fold and the data definition in the lesson, write the following functions:

;; number-of-nodes : Tree -> Number
;; RETURNS: the number of nodes in the given tree.

;; increment-all : Tree -> Tree
;; RETURNS: a tree just like the original, but in which all of the
;; leaves have contents double that in the original.

[ANSWER]