diff --git a/go/algorithm/cartesian/cartesian_test.go b/go/algorithm/cartesian/cartesian_test.go
index 5911ecc..9834483 100644
--- a/go/algorithm/cartesian/cartesian_test.go
+++ b/go/algorithm/cartesian/cartesian_test.go
@@ -1,3 +1,6 @@
+// Copyright The Monogon Project Authors.
+// SPDX-License-Identifier: Apache-2.0
+
 package cartesian
 
 import (