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