diff --git a/grammars/calculator/calculator.go b/grammars/calculator/calculator.go
index 8969470..ecf047d 100644
--- a/grammars/calculator/calculator.go
+++ b/grammars/calculator/calculator.go
@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build grammars
+// +build grammars
+
 package main
 
 import (