diff --git a/gen/gen.go b/gen/gen.go
index ee7c740c..99e6d917 100644
--- a/gen/gen.go
+++ b/gen/gen.go
@@ -1,6 +1,8 @@
 package main
 
 import (
+	"os"
+
 	"github.com/filecoin-project/go-state-types/builtin"
 	gen "github.com/whyrusleeping/cbor-gen"
 
@@ -22,4 +24,15 @@ func main() {
 	); err != nil {
 		panic(err)
 	}
+
+	// now append the string "'// helloworld" to the builtin/cbor_gen.go file:
+	// use regular go api to open the file and write to it:
+	f, err := os.OpenFile("./builtin/cbor_gen.go", os.O_APPEND|os.O_WRONLY, 0644)
+	if err != nil {
+		panic(err)
+	}
+	defer f.Close()
+	if _, err := f.WriteString("\n// helloworld\n"); err != nil {
+		panic(err)
+	}
 }