diff --git a/codebook/1_Basic/black_magic.cpp b/codebook/1_Basic/black_magic.cpp
index b0290a2e..54dbb935 100644
--- a/codebook/1_Basic/black_magic.cpp
+++ b/codebook/1_Basic/black_magic.cpp
@@ -10,8 +10,9 @@ int main() {
   h1.join(h2); // h1 = {1, 2, 3, 4}, h2 = {};
   tree<ll, null_type, less<ll>, rb_tree_tag, tree_order_statistics_node_update> st;
   tree<ll, ll, less<ll>, rb_tree_tag, tree_order_statistics_node_update> mp;
-  for (int x : {0, 2, 3, 4}) st.insert(x);
-  cout << *st.find_by_order(2) << st.order_of_key(1) << endl; //31
+  for (int x : {0, 3, 20, 50}) st.insert(x);
+  assert(st.order_of_key(3) == 1 && st.order_of_key(4) == 2);
+  assert(*st.find_by_order(2) == 20 && *st.lower_bound(4) == 20);
   rope<char> *root[10]; // nsqrt(n)
   root[0] = new rope<char>();
   root[1] = new rope<char>(*root[0]);