code: private static void checkExtraFields(JSTuple tuple, int startCol, int count) throws JMFSchemaViolationException {
      for (int i = startCol; i < count; i++) {
        JMFType field = tuple.getField(i);
        if (field instanceof JSVariant) {
          JMFType firstCase = ((JSVariant)field).getCase(0);
          if (firstCase instanceof JSTuple)
            if (((JSTuple)firstCase).getFieldCount() == 0)
              continue;
        }
        throw new JMFSchemaViolationException(field.getFeatureName() + " not defaulting variant");
      }
    }
docs: Check the extra columns in a tuple to make sure they are all defaultable