Partially implement $(origin)
diff --git a/var.go b/var.go
index ebe1494..2973f91 100644
--- a/var.go
+++ b/var.go
@@ -50,7 +50,7 @@
 type UndefinedVar struct{}
 
 func (_ UndefinedVar) Flavor() string  { return "undefined" }
-func (_ UndefinedVar) Origin() string  { return "" }
+func (_ UndefinedVar) Origin() string  { return "undefined" }
 func (_ UndefinedVar) IsDefined() bool { return false }
 func (_ UndefinedVar) String() string  { return "" }
 func (_ UndefinedVar) Eval(ev *Evaluator) string {