This pattern is like the one for lists, except that it calls itself twice for interior nodes. This is essentially the same as the divide-and-conquer design pattern.
(defun myfun (tree)
(if (interior? tree)
(combine (myfun (first tree)) ; left
(myfun (rest tree))) ; right
(baseanswer tree) )) ; leaf node
(defun addnums (tree) ; sum all numbers in tree
(if (consp tree)
(+ (addnums (first tree))
(addnums (rest tree)) )
(if (numberp tree)
tree
0) ) )
public static Integer addnums (Object tree) {
if ( consp(tree) )
return ( addnums(first((Cons)tree)) +
addnums(rest((Cons)tree)) );
else if ( tree instanceof Integer )
return (Integer) tree;
else return Integer.valueOf(0); }