public void onAttribute( AttributeExp exp ) {
if( exp.nameClass instanceof SimpleNameClass ) {
// this check is only appliable for those who constrains
// one particular attribute.
SimpleNameClass nc = (SimpleNameClass)exp.nameClass;
StringPair p = new StringPair( nc.namespaceURI, nc.localName );
if( atts.containsKey(p) )
throw new Eureka(p); // eureka! : find two AttributeExps that share the same name.
atts.put(p,current);