1 (defun lookup (given-key set)
2   (if (null? set)
3       nil
4       (let* ((cur-entry (entry set))
5 	     (cur-key (key cur-entry)))
6 	(cond ((= cur-key given-key) cur-entry)
7 	      ((< given-key cur-key)
8 	       (lookup given-key (left-branch set)))
9 	      ((> given-key cur-key)
10 	       (lookup
11 		given-key
12 		(right-branch set)))))))