code: private void _reportUndefinedNotationRefs()
          throws XMLStreamException
      {
          int count = mNotationForwardRefs.size();
  
          String id = mNotationForwardRefs.keySet().iterator().next();
          String msg = ""+count+" referenced notation"+((count == 1) ? "":"s")+" undefined: first one '"+id+"'";
          _reportVCViolation(msg);
      }
docs: /*
  /////////////////////////////////////////////////////////
  Internal methods, validation, exceptions
  /////////////////////////////////////////////////////////