int traverse_inorder(struct node *tree, int (*callback)(char *data)) {
    int rv;
    if (tree->left) {
        if (rv = traverse_inorder(tree->left, callback)) return rv;
    }
    if (rv = callback(tree->data)) return rv;

    if (tree->right) {
        if (rv = traverse_inorder(tree->right, callback)) return rv;
    }
    return 0;
}