(define make-leaf (lambda (root) (make-nonempty-tree root (make-empty-tree) (make-empty-tree))))
(make-leaf 1) ⇒ (1 () ()) (make-leaf 3) ⇒ (3 () ()) (make-leaf 5) ⇒ (5 () ()) (make-leaf 7) ⇒ (7 () ())