AVL_XBALANCE
ASSERT(AVL_XBALANCE(node) == 1);
ASSERT(AVL_XBALANCE(node) <= 0);
int child_bal = AVL_XBALANCE(child);
balance = AVL_XBALANCE(gchild);
old_balance = AVL_XBALANCE(node);
old_balance = AVL_XBALANCE(delete);
old_balance = AVL_XBALANCE(node);