diff --git a/tools/warn.py b/tools/warn.py
index 5f796f5..97f54f9 100755
--- a/tools/warn.py
+++ b/tools/warn.py
@@ -27,6 +27,7 @@
 
 
 def main():
+  """Old main() calls warn.warn."""
   os.environ['PYTHONPATH'] = os.path.dirname(os.path.abspath(__file__))
   subprocess.check_call(['/usr/bin/python3', '-m', 'warn.warn'] + sys.argv[1:])