연습문제 2.30

function square_tree(tree){
    return is_null(tree)
            ? null 
            : ! is_pair(tree)
            ? square(tree)
            : pair(square_tree(head(tree)),
                   square_tree(tail(tree)));
}

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