diff --git a/content/data-structures/OrderStatisticTree.h b/content/data-structures/OrderStatisticTree.h
index 8928bc9..b5fe5c7 100644
--- a/content/data-structures/OrderStatisticTree.h
+++ b/content/data-structures/OrderStatisticTree.h
@@ -8,21 +8,16 @@
  * To get a map, change \texttt{null\_type}.
  * Time: O(\log N)
  */
-#pragma once
-
-#include <bits/extc++.h> /** keep-include */
+#include <bits/extc++.h>
 using namespace __gnu_pbds;
 
-template<class T>
-using Tree = tree<T, null_type, less<T>, rb_tree_tag,
-    tree_order_statistics_node_update>;
+// find_by_order(x): return iterator to k-th largest element(count from 0)
+// order_of_key(x): number of element strictly less than x
 
-void example() {
-	Tree<int> t, t2; t.insert(8);
-	auto it = t.insert(10).first;
-	assert(it == t.lower_bound(9));
-	assert(t.order_of_key(10) == 1);
-	assert(t.order_of_key(11) == 2);
-	assert(*t.find_by_order(0) == 8);
-	t.join(t2); // assuming T < T2 or T > T2, merge t2 into t
-}
+typedef tree<
+	int,
+	null_type,
+	less<int>,
+	rb_tree_tag,
+	tree_order_statistics_node_update
+> ost;