code: @Override
      public String validate(DTDValidatorBase v, char[] cbuf, int start, int end, boolean normalize)
          throws XMLStreamException
      {
          String ok = validateEnumValue(cbuf, start, end, normalize, mEnumValues);
          if (ok == null) {
              String val = new String(cbuf, start, (end-start));
              return reportValidationProblem(v, "Invalid enumerated value '"+val+"': has to be one of ("
                                             +mEnumValues+")");
          }
          return ok;
      }
docs: Method called by the validator
  to let the attribute do necessary normalization and/or validation
  for the value.