// BinarySearchTree *bstree_successor(const BinarySearchTree *x) {
//     assert(!bstree_empty(x));
//     printf("succ\n");
//     ptrBinarySearchTree cur = (ptrBinarySearchTree)x;
//     cur = cur->right;
//     while (!bstree_empty(cur) && !bstree_empty(cur->left)){
//         cur = cur->left;
//     }
//     return (BinarySearchTree *)cur;
// }