Design Pattern: Binary Tree Recursion

If we are using the form of binary tree that has contents, left and right, we can alter the design pattern slightly:

(defun myfun (tree)
         (if (not (null tree))
         (combine (contents tree)
                         (myfun (left tree))
                         (myfun (right tree)))
                 (if (test tree) ; leaf
                         (baseanswer tree)
                         safeanswer ) ) )

safeanswer depends on the combine:

combine	safeanswer = identity
+	0
*	1
and, every	true
or, some	false
min	Integer.MAX_VALUE
max	Integer.MIN_VALUE
union	null