diff --git a/graph.go b/graph.go
index f5ad7bb..36e408f 100644
--- a/graph.go
+++ b/graph.go
@@ -1,6 +1,8 @@
 // Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.
 
+//go:build graph
+
 // graph.go
 package expr
 
diff --git a/t_graph_test.go b/t_graph_test.go
index d706d60..2fcad3d 100644
--- a/t_graph_test.go
+++ b/t_graph_test.go
@@ -1,6 +1,8 @@
 // Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.
 
+//go:build graph
+
 // t_graph_test.go
 package expr