diff --git a/book/ate/did.ipynb b/book/ate/did.ipynb
index 65e026d..5feb084 100644
--- a/book/ate/did.ipynb
+++ b/book/ate/did.ipynb
@@ -28,7 +28,9 @@
    },
    "outputs": [],
    "source": [
-    "import pandas as pd"
+    "import pandas as pd\n",
+    "\n",
+    "# comment"
    ]
   },
   {