code: protected void checkEntity(InputProblemReporter rep, String id, EntityDecl ent)
          throws XMLStreamException
      {
          if (ent == null) {
              rep.reportValidationProblem("Referenced entity '"+id+"' not defined");
          } else if (ent.isParsed()) {
              rep.reportValidationProblem("Referenced entity '"+id+"' is not an unparsed entity");
          }
      }
docs: /* Too bad this method can not be combined with previous segment --
  the reason is that DTDValidator does not implement
  InputProblemReporter...