(is-empty-tree? (node '()))
result #t
(is-leaf? (leaf 1))
result #t
(leaf-val (leaf 1))
result 1
(is-leaf? (node (list (leaf 1))))
result #f
(leaf-val (nth-child (node (list (leaf 1) (leaf 2) (leaf 3))) 2))
result 3
(leaf-val (nth-child (nth-child (node (list (node (list (leaf 1) (leaf 2))) (leaf 3) (leaf 4))) 0) 0))
result 1