| (define (lookup key set) |
| (if (null? set) |
| false |
| (let ((e (entry set))) |
| (let ((entry-key (car e)) |
| (entry-val (cdr e))) |
| (cond ((= key entry-key) entry-val) |
| ((< key entry-key) (lookup key (left-branch set))) |
| ((> key entry-key) (lookup key (right-branch set)))))))) |