anchor.diff:     */
     K firstKeyAfter(K key) {
       splay_(key);
  -    K visit(SplayTreeNode node, [K ifEmpty]) {
  +    K visit(SplayTreeNode node, K ifEmpty) {
         if (node === null) return ifEmpty;
         if (node.key.compareTo(key) > 0) {
           return visit(node.left, node.key);
  
positive.code:         return visit(node.right, node.key);
        }
      }
      return visit(_root);
    }
  
    /**
  
negative.code:         return visit(node.right, ifEmpty);
        }
      }
      return visit(_root, null);
    }
  }
  