연습문제 2.31

const x = list(1,list(2, list(3,4),5), list(6,7));

function tree_map(func, tree){
    return map(sub_tree=> is_pair(sub_tree)
                        ? tree_map(func, sub_tree)
                        : func(sub_tree), tree);
}

function suquare_tree(tree){
    return tree_map(square,tree);
}

suquare_tree(x)//[1, [[4, [[9, [16, null]], [25, null]]], [[36, [49, null]], null]]];