diff -r e109b671a19e -r 18b3a9a85716 rt/vm/src/main/java/org/apidesign/vm4brwsr/VM.java
--- a/rt/vm/src/main/java/org/apidesign/vm4brwsr/VM.java	Fri Feb 27 13:50:37 2015 +0100
+++ b/rt/vm/src/main/java/org/apidesign/vm4brwsr/VM.java	Fri Feb 27 18:11:54 2015 +0100
@@ -291,11 +291,7 @@
     
     @Override
     protected boolean requireReference(String cn) {
-        if (references.contains(cn)) {
-            return false;
-        }
-        references.add(cn);
-        return true;
+        return references.addIfMissing(cn);
     }
 
     @Override